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IIS 7.0 Failed Request Tracing (for historical reasons internally we refer to it as FREB, since it used to be called Failed Request Event Buffering, and there are no "good-sounding-decent" acronyms for the new name) is probably the best diagnosing tool that IIS has ever had (that doesn't require Debugging skills), in a simplistic way it exposes all the interesting events that happen during the request processing in a way that allows you to really understand what went wrong with any request.