Zend_Loader, the silent killer
Update 1st May 2009: Good news, Zend Framework 1.8 defaults to no longer suppressing errors in loading classes (not sure how much of their library has been converted to use this though)
A good article on the autoloader changes:
http://devzone.zend.com/article/4525
Several times during website development, I have come across scripts which die without errors, so I try and debug the pages, finding which functions/includes it is getting to before it dies, narrowing it down. The code I usually come across which causes this is in the Zend Framework in the form of:
@Zend_Loader::loadClass('My_Class');