Clinicians are intimately familiar with the pressures of working in a busy, emotionally stressful environment. This commentary uses a framework derived from cognitive psychology to demonstrate the impact of physicians' emotional state on the risk of committing errors. A series of case examples is used to illustrate how subconscious biases, acute stressors, burnout, and overt behavioral disorders can cause clinicians to commit cognitive errors—particularly diagnostic errors—leading to preventable patient harm. Prior research in this area has linked burnout in nurses, resident physicians, and surgeons to increased error rates. The authors offer several suggestions for identifying and minimizing harm among patients and physicians due to emotional stressors; chief among these are early identification of impaired physicians and clinical teaching of metacognition that acknowledges the deleterious effects of clinicians' biases.