When your web server dumps an error message, even something as innocuous as a 404 message it might tell more information than you want. Consider for example this page from a live public site
In this case it clearly indicates you are running IIS. Hopefully you didn’t spend time changing your server fingerprint or masking in other ways because you just gave away the store right here. Already hackers are using error messages to figure out how to find vulnerable IIS boxes on the Web. Using Google it is possible just to search for various error page strings and find some troubling data, take a look at
http://johnny.ihackstuff.com/index.php?module=prodreviews&func=reviewsbycat&reviewsel=10 or just search under "google hacking" and see how potential intruders look at every detail including error message text to figure out how to penetrate your Web app or server.
Bottom line - sanitize your error messages ASAP!