Having just managed to discover the cause of a particular error message, I thought I’d share the solution with the community.
Once in while you come across error messages that point you in absolutely no direction at all. Most recently I’ve been receiving ‘Failed validation: ApexPage‘. Thinking that I might get something more meaningful from the in-browser IDE I dumped my code there and got this little gem
 | Error: common.request.servlet.PageDispatcher$Hack404 | |
| Error: null |
Considering that I was working with a several hundred line file, this made me less than happy. At times like these I whip out that old faithful tool of developer’s everywhere, the binary search algorithm.