We have created http://www.macobserver.com/tmo/404_not_found — That page works.
I then added
ErrorDocument 404 /tmo/404_not_found
to our apache config. Without that line in there Apache delivers its default 404, so I know that logic works. With that line in there, though, I get our home page, NOT the 404 template. It comes in with a 404 header properly, but it doesn't display the right template.
Any ideas why? I've been beating my head on this one before posting here (as we all do, I'm sure :)
(and yes, we renamed our index.php to tmo years ago with EE 1.x)