 even in such highly constrained and high-risk environments as nuclear power plants, modification of instructions is repeatedly found and the violation of rules appears to be quite rational, given the actual workload and timing constraints under which the operators must do their job.In these situations, a basic conflict exists between error as seen as a deviation from the "normative procedure" and error as seen as a deviation from the rational and normally used "effective procedure" (Rasmussen and Pejtersen, 1994).