http://www.google.com/doesnt-exist

Would it -- should it? -- matter if this 404 was generated because the server couldn't find a file or because a script couldn't find a route?

EDIT: A couple other examples.

https://profiles.google.com/doesnt-exist
http://www.facebook.com/reallydoesntexist

Under the hood, it's possible that this is being rewritten to facebook.com/profile.php?slug=reallydoesntexist, but that shouldn't matter. What matters is whether the resource identified by the URL is available. If not, then a 404 Not Found is absolutely the correct response.