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Where did you start to look for it, which way of research have you thought of?
If you want to 'create' a conditional 404 with mod_rewrite, simply internally-rewrite the request to a non-existent URL-path. Or use the [G] flag on a rule to create a 410-Gone response for HTTP/1.1 clients (and later).
[edited by: jdMorgan at 7:11 pm (utc) on April 20, 2008]
The Idea to rewrite to non-existent files I had thought about as well but some errorpages do display the "true" filename of the file then.
The last thing I was working on is to manipulate environment vars sothat apache will display a 404. But I could not proof if this can be done or not.
That will only happen if you use an external redirect instead of a internal rewrite, or if you have a server configuration error that causes a redirect to occur on the request.