Diagnostic errors are an understudied patient safety problem. The emergency department is a particularly challenging environment for diagnosis, due to its fast pace, frequent interruptions, and multiple simultaneous diagnostic trajectories. This study examined voluntary incident reports for diagnostic errors and found that common conditions such as sepsis and acute coronary syndromes were among the most frequently reported as missed or delayed. As with prior studies, the majority of errors involved multiple factors. Cognitive errors and system factors (e.g., inefficient processes and high workload) were prevalent. These results demonstrate the need to address diagnostic safety with both cognitive training interventions and systems approaches.