When implementing a custom 404 ASP page for a client recently I found that the code of the page was displaying as text rather than the parsed code. Accessing the custom 404 page directly however worked without any problems and showed the correct error document.
In the end the problem turned out to be that you can’t specify the page’s “message type” as file if you are using a scripted page such as ASP as it won’t parse the code. If you specify the type as a url and put in the absolute path relative to the sites root however it will work correctly and parse the code. I have included a screenshot below to show the settings a little clearer.
