Increased interconnectivity of Cyber-Physical Systems, by design or otherwise, increases the cyber attack surface and attack vectors. Observing the effects of these attacks is helpful in detecting them. In this paper, we show that many attacks on such systems result in a control loop effect we term Process Model Inconsistency (PMI). Our formal approach elucidates the relationships among incompleteness, incorrectness, safety, and inconsistency of process models. We show that incomplete process models lead to inconsistency. Surprisingly, inconsistency may arise even in complete and correct models. We illustrate our approach through an Automated Teller Machine (ATM) example, and describe the practical implications of the theoretical results.