To identify diagnostic errors caused by faulty clinical cognition, we analyzed 40 consecutive transcripts of problem-solving exercises published in a pedagogic series of clinical reasoning. The analysis disclosed multiple errors in cognition and produced a provisional classification of these errors based on a framework derived from cognitive science. Faults in cognition were identified in all steps of the diagnostic process, including triggering, context formulation, information gathering and processing, and verification. We instantiated each type of error by providing detailed specific examples, and identified the consequences of each error. We conclude that cognitive errors can be identified and classified, that they can produce serious morbidity, and that a classification of cognitive errors is a step toward a deeper understanding of the epidemiology, causes, and prevention of diagnostic errors.