It's the assumption that the hardware operation is the same as the C abstract machine operation. Hint: they're not, and that causes all kinds of subtle problem

It's the assumption that the hardware operation is the same as the C abstract machine operation. Hint: they're not, and that causes all kinds of subtle problem when programmers assume they are, especially when it comes to atomicity. The compiler caches a value in a register and two threads synchronize on that value? Boom. Or maybe no boom when one was expected.