When a webserver cannot find a suitable document to return, it instead sends back a HTTP 406 error with an exhaustive Alternates header containing every varient of the requested URI available. If answering a GET request it also sends back (in Apache's case) a HTML 2.0 document written /in english/. I feel that WebKit could do better here, and use the Alternates header to tell the user what went wrong, generate it's own list of alternates in the user's primary language (as determined from AppleLanguages), and make it look pretty too. rdar://problem/4575352
You could even use UTIs to look up the local name for MIME types, and display "Adobe Photoshop document" instead of "image/psd".
From the Apache documentation on serving custom error messages: Microsoft Internet Explorer (MSIE) will by default ignore server-generated error messages when they are "too small" and substitute its own "friendly" error messages. The size threshold varies depending on the type of error, but in general, if you make your error document greater than 512 bytes, then MSIE will show the server-generated error rather than masking it. More information is available in Microsoft Knowledgebase article Q294807 http://support.microsoft.com/default.aspx?scid=kb;en-us;Q294807
This behavior is not allowed by the HTML Standard and given the mess servers make of status codes I don't think it would end up helping end users.