Individual and system factors have been implicated in past analyses of surgical errors. This prospective study developed an error classification system and applied it to more than 9800 surgical procedures examined. Investigators discovered major complications in 3.4% of cases, with surgical technique, judgment errors, inattention to detail, and incomplete understanding as the most common contributing factors. System and communication errors were infrequently identified, a notable finding given past research suggesting their importance in surgical safety. The authors highlight that their findings challenge "popular wisdom" about the role of systems in patient safety and advocate for greater attention to human factor causes of error.