Philosophers of perception are familiar with the argument from illusion, at least since Hume formulated it to challenge a naïve form of realism. In this paper, I present an analogous argument but in the domain of action. It focuses on slips, a common kind of mistake. But, otherwise, it is structurally similar. The argument challenges some contemporary views about the nature of action inspired by Wittgenstein. The discussion shows how thinking about these common mistakes helps illuminate aspects of human agency (...) that tend to be overlooked whenever too much emphasis is placed on the rationality of human actions. (shrink)