Humans make mistakes every day. In some professions, errors can be shrugged off as costly mistakes. "Geez, it looks like we've installed the processor incorrectly on the last 50,000 widgets. Need a re-do." Nurses, doctors and others in the healthcare know much higher stakes: the tiniest missteps can put a life on the line. Hence, the humble checklist, so beloved by aviators, helps prevent an untold number of disasters.