Just a quick one: Save yourself some trouble and don’t ever name your Drupal module anything that ends on _custom, it’ll save you some headaches. For example getting unexplainable errors like Warning: Illegal offset type in isset or empty in drupal_theme_access () (line 53 in includes / theme.inc) despite being quite sure you did everything right.
This will happen as soon as your module implements hook_theme.
I found this out thanks to Spiderneo blogging (en francais!) about it. Merci beaucoup 🙂