Checklists have been integral components in some of the most notable successes of the patient safety movement. However, the mechanism by which checklists improve outcomes is not entirely clear. A previously published surgical safety system, centered around detailed checklists for the entire preoperative, operative, and postoperative phases, achieved remarkable improvement in surgical outcomes. This study analyzed the checklists themselves to attempt to discern how they improved safety. At least one potential safety incident was intercepted in almost 40% of patients undergoing surgery. The majority of these were detected postoperatively—even though checklist adherence was lowest in the postoperative period. As prior studies have shown that postoperative care quality likely contributes to hospital-level variations in surgical mortality, this checklist's most important effect likely involved facilitating early detection of postoperative adverse events.