Abstract. Interactive systems combine a human operator with a computer. Either may be a source of error. The veri cation processes used must ensure both the correctness of the computer component, and also minimize the risk of human error. Human-centred design aims to do this by designing systems in a way that make allowance for human frailty. One approach to such design is to adhere to design rules. Design rules, however, are often ad hoc. We examine how a formal cognitive model, encapsulating results from the cognitive sciences, can be used to justify such design rules in a way that integrates their use with existing formal hardware veri cation techniques. We consider here the veri cation of a design rule intended to prevent a commonly occurring class of human error know as the post-completion error. Keywords Cognitive architecture, user error, design rules, formal veri cation.