Classically, humans have been perceived as a source of faults in systems. Modern ergonomic views are promoting a somewhat different idea according to which humans are a factor of safety in unexpected situations. The safety of a system cannot be achieved without taking into account these two sides of cognition which compose what is called cognitive flexibility. In this paper, we will consider the cases of a nuclear accident and a plane crash-landing where human cognitive flexibility has impacted the final safety of the system. We aim to discuss the violations that humans have performed in these cases with the assumption that these violations do not always deteriorate system safety. The discussion gravitates around a core argument according to which violations per se do not inform on the safety impairments in a system. Some other dimensions have to be taken into account. We are of the opinion that, among these other dimensions, the accuracy of the operators’ mental model plays a key role, allowing some violations to improve system safety in emergency situations.