Angels and Demons/Divergence from reality
In the beginning of the book, Brown claims that "references to all works of art, tombs, tunnels, and architecture in Rome are entirely factual (as are their exact locations). They can still be seen today. The brotherhood of the Illuminati is also factual." While the book does not specifically say that all of its apparently factual statements are true, Brown's assertions regarding his extensive research have led some readers to take the fictional universe of the book to be congruent to reality.
However, in many ways the fictional universe of the book's storyline significantly diverges from reality.
- Langdon asserts incorrectly that Michelangelo designed the outfits for the Swiss Guards, though this is a common misconception. According to its official site, Commandant Jules Repond (1910-1921) designed the uniforms influenced by the Renaissance artist Raphael.
- Even though they do have a security as well as ceremonial role inside the Vatican, Brown's depiction of the Swiss Guard as a crack special ops squad, along the lines of the SAS, carrying out operations around the city of Rome (outside Vatican jurisdiction) is inaccurate.
- In a flashback, Langdon recalls a lecture he gave in his Symbology 212 class where he tells his class that "The practice of 'god-eating' — that is, Holy Communion — was borrowed from the Aztecs." It's unclear how this could have occurred, as communion has its roots in the Last Supper (ca. AD 30) and the Aztec culture did not rise until the 14th century. Even if the Aztecs had been around when the practice of communion began, there is no evidence of contact between Europeans and the inhabitants of Central America at that time. The first clear evidence of European contact with these people occurred after Christopher Columbus reached the New World in 1492.
- Brown states that Winston Churchill was a Roman Catholic. Churchill was an non-observant member of the Church of England. He was not a "pillar of the church" but a "flying buttress". "I support it from the outside."
- Langdon recalls "that much of Galileo's legal trouble had begun when he described planetary motion as elliptical. The Vatican exalted the perfection of the circle and insisted that heavenly motion must be only circular." In reality Galileo famously refused to believe in his friend Kepler’s elliptical orbits and Galileo's clash with the Vatican was a result of his support for Heliocentrism, that is claiming that the Earth orbited the Sun and not vice versa. Galileo's support for heliocentricity might not have caused such a fuss if the Church hadn't needed to present a unified front against the threat of the Protestant Reformation, and Galileo causing dissent at home in Rome only made the Church respond more finally to the new science.
- Glick explains that George H.W. Bush must be a member of the Illuminati because he was a 33° Freemason. Neither Bush has ever been a Freemason, though both were members of the Skull & Bones fraternity at Yale.
- Brown mentions in a dialogue between Langdon and Kohler that Copernicus was 'murdered by the church for revealing scientific truths'. Copernicus, as a matter of fact, was a Polish priest and was encouraged by other clergymen to publish his findings. He died a natural death, and there is no evidence suggesting that he was murdered.
- The Hashshashin hired by the novel's villains refers to Christians as playing a critical role in the downfall of his sect. This is inaccurate; the Hashshashin were crippled by invading Mongols when their fortress of Alamut in modern-day Iran was destroyed in 1256.
- There is no such publication as the British Tattler. The Tatler, note the spelling, is well known and would not need to be described as "British" to be identified.
- Brown states that the Isola Tiberina was thought to have healing powers ever since 1656. However, the people of ancient Rome had already built a temple to Aesculapius, the Greek god of healing, on the island in the third century BC.
- CERN does not have a private airport or any aeroplanes especially an X-33 spacecraft. In fact, the X-33 was only produced as a sub-scale technology demonstrator and was never actually built full-scale.
- Dan Brown claims that "Bernini's city-wide cross of obelisks marked the fortress in perfect Illuminati fashion; the cross’s central arm passed directly through the center of the castle’s bridge, dividing it into two equal halves." In fact, neither of the two arms of the cross go through the Castel Sant'Angelo. In all likelihood, Brown was using a topographical tourist map to design this plot element and had not realised that the physical layout was different (and such a map would not have been in print for the designers of the city).
- While in the church of Santa Maria del Popolo, Langdon thinks, "Renaissance cathedrals invariably contained multiple chapels, huge cathedrals like Notre Dame having dozens." This leads the reader to believe that the architecture of the Notre Dame cathedral in Paris is of the Renaissance period. In fact, Notre Dame is the most famous example of Gothic architecture in the world, which pre-dates the Renaissance period. (Brown also seems to assume that all churches in Rome are cathedrals. The term "cathedral" is reserved for those churches where a bishop has his throne (or "cathedra"). The one (Roman Catholic) cathedral in Rome is St John Lateran--and not St Peter's in the Vatican).
- A straight line extending south west of Piazza del Popolo does not pass through the centre of St. Peter's Square, nor does it even pass through Vatican City.
- The Saint Peter's Basilica's crypt (Grotte Vaticane), where most Popes are entombed, is open to the public.
- The BBC headquarters is not (as the novel states) located in Piccadilly; the main BBC headquarters is Broadcasting House in Langham Place, just north of Oxford Circus, and there are also headquarters buildings for the BBC World Service (Bush House, in Aldwych) and for BBC Television (the Television Centre, in White City). Although the latter is in a sense "west of Piccadilly Circus", it is several miles to the west, not immediately west as the novel's wording implies.
- Brown claims that hatha yoga was an ancient Buddhist art. Although influenced by Buddhism, Hatha Yoga pre-dates Buddhism. It was, and is, a Hindu art.
- In examining the body of the previous Pope, the Swiss Guard simply slide back the marble top of the tomb and they check the tongue of the Pope. In fact, Popes are buried in three nested coffins, which would have made this task somewhat more difficult.
- The novel asserts several non-facts about the process of a Papal conclave. It is suggested that only cardinals may be elected in a conclave. This is not true, any male Roman Catholic may be elected. It is further suggested that four candidates are semi-formally chosen, including a head, to become the candidates for papacy, thus making the Conclave obsolete. This is again not true; although there are papabili (generally, cardinals considered to have a chance of being elected), there are no formal candidates before the beginning of the conclave. Further, ballots are restricted to two in the morning and two in the evening, each group is burned together. Dan Brown indicates they are burned individually and indicates that more than two may occur in a single evening. The ballots are read out by one cardinal, verified by another, and pierced by a third; the book suggests that a single person does all of these. The Cardinals do not stay in the Sistine Chapel overnight. They go back to Domus Sancta Marta after two ballots in the morning and two ballots in the evening (in the case a pope hasn't been elected in those ballots). Brown gives the role of camerlengo to the late Pope's secretary. The camerlengo does organize the Conclave but he is never the pope's secretary. In fact, the camerlengo is a member of the College of Cardinals and is pre-chosen by the pope to take charge of the affairs during the Sede Vacante. The late Pope's secretary has no authority over anyone in the Vatican, as he loses his job when the pope dies. Also, the book states that the "Dean of the College of Cardinals" of the conclave cannot be elected; in fact, he can, and he actually was in the 2005 conclave (then Cardinal Ratzinger was the Dean of the College of Cardinals). It should also be noted that the "Devil's Advocate" has nothing to do with the papal election but with the process of beatification. Langdon's encounters with the camerlengo take place in the Papal Apartments, while in fact the rules order that the apartments be sealed during the Conclave and the late Pope's staff has to live somewhere else.
- At the end of the novel, Glick reports that the camerlengo was elected pope for a short time through something called "election by adoration." This is probably meant to refer to the process of election "by acclamation or inspiration" (Latin: per acclamationem seu inspirationem), which was explicitly prohibited as a form of Papal election in 1996, when Pope John Paul II released the apostolic constitution Universi Dominici Gregis. This document also forbids the election of a pope by compromise or committee (Latin: per compromissum), leaving only the secret balloting of conclave as a legitimate form of election.
- Concerning Cardinal (Kardinal) Ebner from Frankfurt (the first victim): There are two major cities in Germany that are called Frankfurt (in addition to small one(s)): Frankfurt am Main and Frankfurt an der Oder. None of them are one of Germany's dioceses and therefore none of the Frankfurts have a bishop. It is not a necessity to be a bishop in order to be chosen to become a cardinal, but that is of course with absolute majority the case. In Germany there are two theologians who became cardinals (the rest were bishops): Alois Kardinal Grillmeier and Leo Kardinal Scheffczyk. So theoretically it is possible that there is someone from Frankfurt who was chosen to become a cardinal. But more likely is that Dan Brown made a rather common mistake: Frankfurt am Main has a big church in its center called "Dom" (cathedral). A cathedral (German: "Dom" or "Kathedrale") is the central church of a diocese and the bishop's seat. Therefore it is not uncommon that people think Frankfurt is a diocese and has a bishop. But in this case the term "Dom" was used as a general description of a large and historically important church. The name of the Frankfurt Dom is "St. Bartholomäus" (Bartholomew) (see: Kaiserdom St. Bartholomäus). Everyone refers to that church though as "Dom" or "Kaiserdom" (Emperor's Cathedral), because from 1562 to 1792 ten German emperors were crowned in that church. Strictly speaking though the use of the word "Dom" is wrong.
- Cardinal's cassocks are not black as those of ordinary priests, but scarlet.
- Brown refers to the Biblical Book of Judges as the Book of Judgments.
- In a flashback in the novel, the late pope confesses to Carlo Ventresca that he, the pope, has a son. Ventresca, waiting for no further explanation, rushes out, distraught at his mentor's broken vows (see "vows" below). Later in the novel, Cardinal Mortati tries to reassure Ventresca that the pope's love (and that of his beloved, the nun, Sister Maria) was chaste, since Ventresca was conceived without sexual intercourse, by means of artificial insemination, thereby ensuring that both parents remained faithful to their vows. This explains the former pope's love of and support for science, since it enabled him to remain a celibate priest whilst becoming a father. Artificial insemination is forbidden by the Catholic Church and is considered sinful, since it separates conception from the sex act, and also (usually) requires masturbation for the gathering of sperm.
- Strictly speaking, nuns, monks, friars, etc (i.e., all those consecrated to religious life), take "vows" to God, while diocesan clergy make "promises" (including celibacy), to their bishop.
- Italian Laws do not allow singles to adopt a child; a catholic priest like Leonardo Vetra could not adopt Vittoria as a daughter giving her his surname.
- The proton is not, as stated in the book, the antiparticle of the electron. The proton is the antiparticle of the antiproton; the electron is the antiparticle of the positron.
- The process of creating antimatter described in the book is inaccurate. Instead of colliding two particles moving in opposite directions, CERN smashes protons into a stationary block of copper or iridium.
- It is impossible to create and sustain the densities of antimatter described in the book. In fact, it takes 2 billions years to create that much of antimatter. Similarly, antimatter could never be used as a source of power. Antimatter has to be created since it is found nowhere on Earth naturally. The amount of energy required to operate the particle accelerator would be far greater than (or, ideally, equal to) the energy the antimatter-matter reaction would produce. To produce antimatter with less energy than it released would be in violation of the 1st law of Thermodynamics, so even a "ground breaking discovery" as described in the book occurred, it would not be physically possible.
- Brown writes that the antimatter canister will detonate with the energy of a 5 kiloton bomb. However, the energy released will be from the total consumption of 0.5g of material; 0.25g of antimatter, and 0.25g of normal matter. This has the energy equivalent of about 1013 Joules, or about 10 kilotons of TNT. The Hiroshima bomb was about 20 kilotons. The Nagasaki bomb was 13 kilotons.
- Containment of hydrogen (anti- or otherwise) in a magnetic field would not be possible in a small portable container. For the hydrogen to form a droplet at normal temperatures and in a vacuum (as would be necessary to isolate it from normal matter) would require a magnetic field of extraordinary strength, beyond the power of a portable power source. This magnetic field would have to extend well beyond the canister and would cause vigorous effects on ferromagnetic materials nearby. Furthermore, hydrogen in its conventional liquid state is diamagnetic, and would be difficult to suspend unless the compression induced by the magnetic field caused the hydrogen to become metallic (as in the core of gas giant planets).
- Having jumped out of a helicopter seconds before a ten kiloton antimatter annihilation, Camerlengo Ventresca would most likely have been vaporised. If not, then the explosion would have propelled him at such a speed that there would be absolutely no chance of his slowing enough to land safely.
- Annihilation of protons and anti-protons would generate energy primarily in the form of 900 [MeV gamma rays. These gamma rays would easily penetrate the matter surrounding the bomb and deliver a lethal dose of radiation to any exposed person standing closer than 2-3 km from the point of explosion.
- Near the end of the novel, Brown repeats the popular notion that humans only use a small percentage of their brains. In fact, this is a myth. 
- The pilot explains Langdon's nausea is altitude sickness from flying at 60,000 ft (18,288 m). "Altitude sickness" is not due to altitude but to (lack of) air pressure. A properly pressured aircraft should not induce altitude sickness. Many Concorde passengers flew at 60,000 ft without experiencing altitude sickness.
- Rather than being the mature transport system described in the book, the Lockheed Martin X-33 is, in fact, a small unmanned technology-demonstration vehicle. Intercontinental hypersonic spaceplanes are planned, but none exist.
- Arguably the most blatant and central error made in the book concerns the wireless camera which occupies a central role in the search for the antimatter canister. In the book, the canister is supposedly being filmed by a wireless camera and that signal is being displayed on a large screen in St. Peter's square. In order to find this camera, the Swiss Guard needs to shut off every electronic device to and conduct a painstaking search for the camera, which is eventually found, with the canister, deep underground in a labyrinth of tombs beneath the Vatican. There are several problems with this plot line.
- First, if the wireless camera's signal is being received and the picture displayed in St. Peter's Square, then it would not take more than a few minutes for a trained team of engineers to triangulate the position of the transmission. Indeed, consider the countless movies about World War II clandestine radio operators who could not transmit more than a few minutes at a time so as not to be detected by the counter-intelligence triangulation teams. Here we have a camera that is transmitting for hours. The rule is: if you can receive the signal, it is very easy to find out where it comes from.
- The other, second and perhaps greater error is the fact that this wireless camera is transmitting from underground. Transmission of a video signal takes a lot a bandwidth and hence must have a relatively high carrier frequency (such as VHF, UHF, or SHF). In the book we are told that we can see the canister's display counting down in seconds in real-time, which means that we are indeed talking about a relatively high bandwidth signal, and hence, the assumption of a high carrier frequency is a good one. Such carrier frequencies cannot penetrate any significant obstacles and hence it is simply impossible for the signal of such a camera to be received if it were transmitting from deep underground.
In conclusion, the whole premise of the plot, namely a laborious search for a canister which is being filmed by a wireless camera from deep underground, does not stand the scrutiny of basic radio transmission principles.
- There is the lack of a method for recharging the anti-matter canister. A container requiring electrical power to function designed both to be easily portable and to hold such quantities of dangerous material would have the option to use an alternate method to recharge the on-board battery, preferably one that can be as easily employed as a standard "wall wart" power adapter and an electrical outlet. Without a similar option, the canisters themselves are shockingly unsafe for their stated function from an engineering standpoint and pay little heed to Murphy's Law.
- The author appears to confuse the invention of the World Wide Web by Tim Berners-Lee at CERN in the 1980s with the invention of the Internet mainly in the US in the 1960s. Far from being unrecognised for his work as is implied in the book Tim Berners-Lee has received numerous awards including appointment to the Order of the British Empire as a Knight Commander (KBE). He is also head of the W3C which manages and defines the most popular WEB standards.
- Cellular phones do not have dial tones. Furthermore, cellphone calls cannot be made untraceably as suggested in the book; whenever a cellphone is switched on, even if it is in standby mode, the service provider can detect which cell it is in and hence where it is to within a few metres.
- "Illuminati: New World Order", the game mentioned in the book, is a Collectible card game, not a computer game. Steve Jackson Games is not a computer game company. It is not playable on the internet, and one does not score points by killing opponents.
- Gunther Glick, the supposedly "British" journalist, has speech and thoughts which are riddled with Americanisms (e.g. "gas" instead of "petrol", "anchormen" instead of "presenters"), and in a fantasy about future success, he likens himself to Dan Rather — who is relatively unknown in Britain. A real British journalist would probably most likely liken himself to Trevor McDonald.
- Langdon states at one point that the phrase Novus Ordo Seclorum, found on American currency, translates as "New Secular Order." The correct translation is "New Order of the Ages."
- Langdon tells Kohler that the Church declared the Illuminati "Shaitan," which he identifies as an "Islamic word." In fact, the word "Shaitan" (Arabic شيطان) comes from ancient Hebrew "ha Satan," (standard Hebrew שָׂטָן In Hebrew, Shaitan translates to "adversary". In Islamic culture, the word refers to anyone who has rebelled against God.
- Robert Langdon incorrectly translates Seraphim as "The Fiery One". In reality, Seraphim is the plural form of Seraph. This is a commonly made error.