The predictive processing theory of action, cognition, and perception is one of the most influential approaches to unifying research in cognitive science. But it is somewhat difficult to say how to test this theory empirically and whether it is even possible at all. In this paper, we argue that principles, general theories and particular models driven by PP can be only partially tested or falsified. In some cases, there is no sufficient detail to make falsification possible, and when details are present, the models, theories and principles seem often implausible or outright false. Moreover, if one assumes that proper explanations should be mechanistic, several important lessons for PP can be drawn. As we will argue, current research practice is nonetheless far from the normative ideals for mechanistic explanations and cognitive modeling in general.