i was having problems all day with what seemed to be 404 misfirings.
anyway, i just finished chatting with a dreamhost tech support person who told me that a user account i have with them was hitting process memory resource limits (all processes) and that was what was causing strange, seemingly htaccess-related problems. i was getting intermittent 404 errors from an htaccess file that should not have been called at all! it was dreamhost with a haunted house server.
apparently, the process killing robot that dreamhost uses will kill a web process in the middle and then for some reason, the (now zombie) apache actually tries to finish its job (doing its best to exit cleanly out of the unglamorous end to a subrequest is my best guess). it throws a 500 error into the main http log, but after doing so, it actually fires off the rewrite condition and rule that should never have been fired (using the standard file -f and directory -d htaccess file above) – and it doesn’t write a new log entry! a new (invisible man) request then triggers the index file in the last line of the htaccess file
beware the resource limits in dreamhost basic accounts! if you go over their limits, and you have htacess with mod_rewrite lines you will see strange things that are only fit for halloween night – invisible men, haunted 404s! undead processes! zombie apache! htaccess moving on its own! yikes!
hope this helps you avoid some hours of pain.