The autoload mechanism doesn't order of active extensions. This is a problem when you want to create an extension that override a class of another one.
The autoload will load the first class that is met (alphabetical order I think) and this maybe not be the class you want to be loaded.

Steps to reproduce

On a fresh install. Copy a class from an extension that is first in alphabetical order to an extension that is first in active extension.

The autoload will load the one that is first in alphabetical order.