Getting trace data from umbraco 
You can get a complete stack trace from umbraco, by appending "?umbDebugShowTrace=true" to the querystring. This include data on all actions inside umbraco from the initial request to the final response, including what page data has been loaded and what data has been sent to macro parameters.
Getting data sent to macro elements 
You can get see a list of macro elements sent to an XSLT macro by appending "?umbDebug=true" to the querystring
Production systems 
This feature can be turned off for production systems by setting umbracoDebugMode to false in web.config.
The following example is from ISAPI rewrite's http.ini:
RewriteRule /.*umbDebugShowTrace.*$ /default.aspx RewriteRule /.*umbDebug.*$ /default.aspx