We investigated effects of task interruption on procedural performance, focusing on the effect of interruption length on the rates of different categories of error at the point of task resumption. Interruption length affected errors involving loss of place in the procedure (sequence errors) but not errors involving incorrect execution of a correct step (nonsequence errors), implicating memory for past performance, rather than generalized attentional resources, as the disrupted cognitive process. Within the category of sequence errors, interruption length produced a complex pattern of effects, with repetitions of the preinterruption step showing different effects than errors at other offsets from the correct step. A cognitive model we developed previously accounts for the results in terms of decay and rehearsal of memory for past performance and activation spreading through a procedural representation of task knowledge. The model links different types of errors to different cognitive processes, informs potential interventions, and predicts interruption effects for sequential tasks like problem solving and counting.