I wasted a few hours on figuring out why an init hook in one of my modules wasn't invoked when normal page caching was on. Turns out the boostrap field in the system table for my module was still 0, instead of 1. So remember boys and girls, when you allready enabled your own custom module and you add an hook_init, remember to disable and enable your module again so Drupal knows it exists. Or manually alter the record in the system table in your database if disabling the module breaks your site.

On the bright side, I think I know every line of bootstrap.inc and cache.inc file now by heart.