Why Spring makes me sick #10

Spring uses BeanPostProcessors to make it possible, to modify objects when they are created and initialized.
So far so bad (bad because the unsuspecting developer may never find out what messed with his class, because the the actual BeanPostProcessor instances don't even need to have a reference to the types they are modifying).
But what is really, really bad, is that the BeanPostProcessors (may) even return different objects i.e. you configure one type of bean class and actually get a different class at run time. And all automagically in the background with no way to trace it.