I do a lot of my browser testing (client side for web applications) using Firefox. Whenever I experience an unexpected result in Firefox is when I check with IE. Internet Explorer has a feature that intercepts error-encoded messages and displays a user-friendly error page that hides the raw text of the error - useless when you want to debug with the browser. It'd be helpful if the "More Information" option actually showed the error, but it simply restates what is already mentioned elsewhere on the page. Dumb.
To disable this feature, open Internet Options -> Advanced Tab and uncheck the "Show friendly HTTP error messages" option. If you troubleshoot page display errors across a network, it might help to have this option cleared in group policy so it's distributed to all organization browsers. Or you could use the IEAK.
I've had to do this multiple times when users say they can't access pages and an error page is what appears.