If you run IIS, are you familiar with its HTTPERR logs? If not, you may be missing out on some useful diagnostic information. Sadly, many (sometimes most) of the entries in the logs are innocuous (you don't need to worry about them), but sometimes they're useful. And if have noticed the logs, perhaps you'd like to know more about them.
There's a useful MS document, Error logging in HTTP API, with more about the HTTPERR logs, including their location, format, and info on the kinds of errors reported within them. Hope that's helpful to my readers.
Here's another (more brief) introduction to the files: in the technical reference section of the IIS 6 docs.