If no browser displays your custom 404 messages, then your server
probably is not configured properly.
If only Internet Explorer ignores your custom 404 messages, then
you've been caught by its "friendly" HTTP error messages. When a
special HTTP response (e.g., a 404 response) is shorter than 512
bytes, Internet Explorer substitutes its own message for the one
delivered by the server. As a user of Internet Explorer, you can
disable this feature in the "Advanced" options panel. As a web
author, your only recourse is to make the error page longer.
>I am using a similar idea to redirect 404 errors to my error-
>recovery page. But I am not sure how to learn which pagerequest
>created this error so that I can avoid the error itself.