Here is a list of some IMPORTANT hacks to applies on your Joomla! installation to fix common issues.

The forked XML does not recognize sub items.

libraries/joomla/forms/forms.php - LINE 723

Fatal error: Access to undeclared static property: XxxxCkClassLoader::$classes

This normal error raises with Joomla 1.6
Since 2.6.3, Cook overrides the JLoader class and need to access to the local properties.
For Joomla 1.6 it is nessessary to apply this patch to make the last 2.6.3 generated component working.

(Also, you can change the config, choosing 'Legacy loader')

libraries/loader.php - LINE 20

Change private to protected