Desperately underused HTTP error code. Many broken links that return a 404 really ought to return a 410 instead.

From the HTTP 1.1 spec: "The requested resource is no longer available at the server and no forwarding address is known. This condition is expected to be considered permanent. Clients with link editing capabilities SHOULD delete references to the Request-URI after user approval."

Gone

The requested resource
/path_to_file_or_directory
is no longer available on this server and there is no forwarding address. Please remove all references to this resource.

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It's pretty easy to cause a 410 - simply insert the following line in your .htaccess file:

Redirect gone /path_to_file_or_directory

Apache will then block access to the file or entire path. And it probably confuses people, too, since they won't have seen such an error before...