Lighttpd

mod_rewrite configuration does not allow disabling rewrite if URL points to existing file or directory.

Basically if URL matches regex it is rewritten - it makes rewrite rules overly complex in certain cases. Good example: Joomla with any 3rd party SEF - to have a correct rewrite rules you've got to match every single directory and rewrite it to itself making 13 rewrite rules instead of 1.