This is the fundamental problem: When you allow the OS (or the compiler, or the CPU) to re-order operations in the name of efficiency you lose control over intermediate states, and so you cannot guarantee that these intermediates states are consistent with respect to the semantics that you care about. And this is the even more fundamental problem: our entire programming methodology has revolved around describing what we want the computer to do rather than what we want to to achieve. Optimizations then have to reverse-engineer our instructions and make their best guesses as to what we really meant (e.g. "This is dead code. It cannot possibly affect the end result. Therefore it can be safely eliminated.) Sometimes (often?) those guesses are wrong. When they are, we typically only find out about it after the mismatch between our expectations and the computer's have manifested themselves in some undesirable (and often unrecoverable) way.