The authors identified 258 malpractice claims from 4 liability insurance companies where patients were harmed due to surgical error and reviewed these cases to determine the relative contribution of 17 "human factors" to the adverse event. Both individual and system factors contributed to errors, classified as cognitive errors, lack of technical competence or knowledge, communication breakdowns, patient-related factors, and others. The cases resulted in significant harm: 23% of the patients died, and 65% suffered disabling injuries. Most cases involved more than one clinician, and 31% involved multiple phases of care (eg, intraoperative and perioperative). The authors recommend researching methods to improve outcomes for less experienced surgical teams and reducing communication errors through structured signout and communication systems.