[en] The introduction of rules and procedures to guide front-line operators’ behaviour and to decrease the frequency of errors is a growing safety strategy in complex, risk systems. It is thought to be a useful way of controlling and standardising human practices and of increasing safety and quality. However, merely developing procedures does not ensure that they are followed. In this study, observation was used to collect information on procedural violations in a pharmaceutical company. Interviews were conducted with the operators and the prescriptors to better understand how and why these violations were occurring. Results showed that a small number of procedures were breached by the majority of operators and that the rules which were violated were the ones associated with a perception of minimum risk. Results suggest the rationality of operators is a response to cognitive and social influences which must be taken into account when designing procedures.