A postcompletion error (PCE) is a specific kind of cognitive slip that involves omitting a final task step after the main goal of the task is accomplished. It is notoriously difficult to provoke (and hence study) slips under experimental conditions. In this paper, the authors present an experimental task paradigm that has been shown to be effective for studying PCEs in routine procedural tasks. Two studies were carried out to examine the effect of interruption position and task structure on the prevalence of PCEs. It was found that significantly more PCEs were obtained when an interruption occurred just before the PC step than when an interruption occurred at any other position in the task. The authors account for this effect in terms of Altmann and Trafton's activation-based goal memory model. The same interruption effect was obtained for some, but not all, other procedural errors; the authors discuss the nature of these errors and likely explanations for the differences.