ZF-8158: Autoload bug with class_exists
In several places (I've only inidicated ones I've run into), class_exists() is called prior to calling Zend_Loader, however, it's called without overriding the default autoload behaviour, and is thus likely to interfere with autoload behaviour happening outside the framework and disable the internal use of Zend_Loader for Zend components. This was introduced in Zend framework 1.8.2. Various instances of this approach have since been fixed as bugs, such as http://framework.zend.com/issues/browse/ZF-6660 and http://zendframework.com/issues/browse/ZF-6591 so I think these should be fixed too.
I suspect that all uses of class_exists should be checked.