If you're hacking on a Drupal module and you write ANY function that has the word theme in it make sure you

CLEAR THE FUCKING THEME REGISTRY CACHE

I just wasted 4 hours trying to figure why no theme function I wrote had any affect on anything.

The more I hack on Drupal the more I realize that it really needs a Rails-like separation between Production and Development environments. Caching should NEVER be enabled when your developing unless you're specifically working on caching.