500 Internal Server error for all ASP pages
If you're using IE5 and/or developing on Windows 2000, you might find it difficult to debug ASP errors in a browser. This is because IE5 has a ridiculous default option that suppresses errors to a more "friendly" error (which is a lot more cryptic than what they'd get otherwise).
To circumvent this silliness and get real ASP errors, go to IE's
Tools/Internet Options menu, and on the advanced tab, uncheck "Show friendly HTTP error messages."
After you've disabled this default setting, refresh the page in question.
There are three possible outcomes: (1) the page will magically work again; (2) the page will give you a more detailed error (e.g. Stack Overflow or Syntax Error), including a line number; or (3) you will get "Server Application Error" - which means that at some point IIS got confused about the current application.