The Rogue Bludger
Chapter 10 of Harry Potter and the Chamber of Secrets: The Rogue Bludger
Following the rather disastrous Pixies episode, Professor Lockhart now mostly re-enacts scenes from his books with Harry's unwilling assistance. Today Harry is being a Werewolf, and he plays along only to stay on Lockhart's good side. After class, Hermione asks Lockhart to sign a note for a library book to help her understand something in one of his works; he readily signs it, then offers to provide Harry advice on being a Quidditch Seeker. Madam Pince somewhat reluctantly accepts the note, and gives Moste Potente Potions to Hermione.
Shortly thereafter, Hermione, Harry, and Ron gather in Moaning Myrtle's bathroom where they are unlikely to be disturbed. Hermione reports that she needs Boomslang skin and Bicorn horn for the potion, ingredients unavailable in the student supplies cupboard. The potion will take about a month to brew, and Hermione, surprisingly, not only is willing to take some risks, but convinces Harry and Ron that they must also.
Just before the Quidditch match against Slytherin starts, Wood assures the team they can beat Slytherin, despite their faster brooms. He tells Harry to do whatever he can to catch the Snitch. As the match is underway, a rogue Bludger immediately targets Harry. Despite Fred and George's best efforts to block it, the Bludger seems determined to knock Harry off his broom; as the team is being slaughtered by the other Bludger and the Slytherin Beaters, Harry tells Fred and George to only concentrate on the game, he can handle the rogue. Malfoy, meanwhile, is so intent on sneering at Harry's frantic efforts to dodge the rogue Bludger, that he fails to notice the Snitch is right by his head. Harry spots it and takes off, but as reaches for the golden ball, the rogue Bludger smashes into his arm, breaking it. Harry catches the Snitch with the other hand, landing hard on the ground intact, then faints. He regains consciousness to see Lockhart standing over him. Lockhart promptly casts a "mending" spell to repair Harry's arm, but it instead removes all the bones.
In the hospital wing, Madam Pomfrey is furious—she can easily fix broken bones, but regrowing them will be a painful, overnight job. The Gryffindor team arrives to celebrate the win over Slytherin but Madam Pomfrey promptly ejects them.
Later that night, Harry is awakened by Dobby sponging his forehead. Dobby admits that he closed the barrier at Kings Cross station to prevent Harry returning to the school, and also enchanted the Bludger, hoping Harry would be injured enough to be sent home. He explains that a House-elf can only be freed if his Master gives him clothing, which is why he wears a ragged pillowcase. He also mentions that the Chamber of Secrets was opened once before, though he punishes himself for having revealed something he ought not. Hearing a noise, he vanishes. Professor Dumbledore and Professor McGonagall bring in a Petrified Colin Creevey. Colin's camera is completely melted inside; Dumbledore says this proves that the Chamber is open again, but the main question is not who, but how?
Lockhart's magical ineptness is becoming more apparent. Now, rather than teaching self-defence charms, Lockhart merely re-enacts scenes from his books, seemingly to avoid another Cornish Pixies-type incident. And, despite his claim he is adept in healing magic, his attempt to repair Harry's arm only results in a worse injury. One must wonder why Hermione, normally so logical, again rejects any suggestion that Lockhart is incompetent. It is perhaps telling that after Ron rhetorically asks aloud which teacher would be thick enough to grant a second-year student permission to check out a Restricted book, that Hermione chooses Lockhart to obtain just such a signature. Could she, despite her infatuation, perhaps now see that he is not quite as brilliant as he claims? Does she believe it is easier to hoodwink a new teacher rather than one who has been at Hogwarts much longer? Or is she simply becoming aware of his vanity, and chooses to play on that to get what she wants, while still retaining some belief in his competence?
The rogue Bludger initially seems to confirm Dobby's warning that Harry is in danger. Only after the game is it learned that it was Dobby who unleashed it, in an effort to convince Harry he should leave Hogwarts; Dobby knows the Chamber was opened relatively recently and that Harry's life is in peril. His misguided attempts to keep Harry away from Hogwarts have been laughable and child-like. Dobby first tried tricking Harry into believing his friends no longer cared about him by intercepting their letters. When that failed, he created havoc in the Dursley house, though he never predicted the fallout: the Ministry of Magic's owl and Uncle Vernon locking Harry in his room, resulting in Ron and the Twins coming to Harry's rescue. Dobby's blocking the barrier to the train station platform also failed when Harry and Ron instead took the Flying Car to Hogwarts, and now this latest attempt to injure Harry badly enough to send him home has similarly failed. All these acts seem poorly planned and executed. Does Dobby actually believe that Muggle medical care is better than what Hogwarts can offer? Or does he believe that Uncle Vernon and Aunt Petunia would allow magical healing in their house? Dobby seems oblivious that Harry's house is not his home, and that Hogwarts, no matter how dangerous it might be, is far more welcoming than Privet Drive.
The Quidditch match is revealing in other ways. Here Slytherin again shows their true nature. Whereas Gryffindor has trained long and hard to win by skill and fair play, Slytherin not only relies on the superior brooms Lucius Malfoy provided the team, but they also use the rogue Bludger to their advantage, easily racking up more points as Gryffindor's defences are divided by fending off both the Slytherin team and the menacing ball that is attacking Harry. Any other team would probably have halted the playing until the rogue was eliminated, then restarted the game, but Slytherin exploits any opportunity. Draco, meanwhile, rather than focusing on being a team member and helping Slytherin win, is instead so pre-occupied with himself and jeering at Harry as the rogue Bludger chases him, that he is oblivious to the Snitch hovering next to him. We suspect that the Slytherin team captain will have some rather harsh words for Draco following Harry's winning the match due to Draco's inattention.
Despite its being placed at the very end of the chapter, in a position of prominence, it is far too easy to overlook Dumbledore's statement that the operative question about the re-opening of the Chamber of Secrets is not who, but how. This should indicate, to the attentive reader, that Dumbledore knows the identity of the Heir and is only wondering how he managed to get the chamber re-opened.
- Why does Hermione select that particular bathroom to brew the Polyjuice potion in?
- Why did Lockhart choose Harry to participate in the class demonstration?
- Knowing what we do about Lockhart, why did he insist on "repairing" Harry's broken arm?
- Who charmed the Bludger to attack Harry, and why?
Further Study 
- Why did Hermione choose Professor Lockhart to sign a permission slip to check out a restricted book? Why does he sign it?
- Why does Dobby tell Harry that he has been trying to force him to leave Hogwarts? Why does he then punish himself for revealing this?
- What does Dumbledore mean when he says it not important to know "who" opened the Chamber of Secrets, but rather "how"?
- What does the Slytherin team's actions in the Quidditch match say about Slytherin House in general?
- What does Draco's performance as the Slytherin Seeker say about him?
Greater Picture 
After stating that the Chamber of Secrets has been reopened, Dumbledore comments that the question should not be who opened it, but how. This implies that Dumbledore rejects the commonly-held belief that it was Hagrid who opened it before, and that Tom Riddle caught him in the act and prevented it. If Dumbledore suspects Riddle opened the Chamber fifty years ago, and believes he has reopened it now, Dumbledore may have already formulated theories about Horcruxes. Until the Diary is revealed, any such theory would be tentative, though any suspicions will be further bolstered in the next chapter when it is revealed that Harry is a Parselmouth. This extremely rare ability to speak to snakes is shared by Voldemort's family, and Dumbledore will rightly surmise that Harry did not inherit this skill naturally. Dumbledore no doubt also remembers the previous year when Voldemort's shade was riding Professor Quirrell, and he may be wondering if the Dark Lord has found another mount, despite rumours placing him in Albania.
It seems Dobby understands the Diary's nature better than Lucius Malfoy, who sent it into the school as a weapon. We will learn that Hogwarts employs several hundred House-elves, and House-elf society likely communicates amongst themselves. It is possible that even if Dobby does not know what the Chamber of Secrets conceals, he is at least aware that Lucius has provided the means to reopen it, fifty years after its previous opening when a student died. It is almost certainly fear for Harry's life that causes Dobby to employ these feeble attempts to return Harry "home" to Privet Drive.
Having revealed that he tried to force Harry to leave Hogwarts, and having been fiercely rebuffed, Dobby will not make any further attempts. Dobby is likely aware that attempting to trick Harry again is useless, as he will immediately be suspected, rendering any action on Dobby's part ineffective.