In Drupal 7 we have a very nice "registry" that registers what PHP classes should be loaded from what files.
Unfortunately you can get the system into a state where it can't bootstrap to a high enough level to rebuild the registry (normally done by clearing the cache). What do you do then?
I had this situation today due to an annoying Rules/Entity issue, and dereine pointed out that it had recently been an issue in the Views queue.
Since the solution given in the Views queue didn't work for me, I created a package that you can use to get out of this trouble, Registry Rebuild.
When would you need Registry Rebuild?
You might get something like:
PHP Fatal error: Class 'EntityAPIControllerExportable' not found in ...sites/all/modules/rules/includes/rules.core.inc on line 11
If this happens when you're trying to run update.php, and happens when you're trying to clear your cache, well, you have some trouble. That's what Registry Rebuild is for.