According to Gopal Vijayaraghavan, PHP developers "don't really understand PHP":
They know how to use PHP - but they hardly know how it works, mainly because it Just Works most of the time. But such willful ignorance (otherwise known as abstraction) often runs them aground on some issues when their code meets the stupidity that is APC.
To illustrate, he talks about how a simple bit of functionality in PHP, includes, really work. He describes how they're handled (evaluated at run time) and how the APC relates to this process.
He presents a dilemma where classes are created in two different ways (inside and outside included files) and a problem that could cause the APC caching to not cache a file at all after a certain point. He even gives an example of how, in "normal code" and a pretty basic development situation, this sort of thing could happen.