Improving diagnosis is a patient safety priority. Using data from patient safety incident reports, malpractice claims, morbidity and mortality reports, and focus group responses, this study sought to identify “diagnostic pitfalls,” defined as clinical situations vulnerable to errors which may lead to diagnostic errors. The authors identified 21 generic diagnostic pitfall categories involving six different aspects of the clinical interaction – diagnosis and assessment, history and physical, testing, communication, follow-up, and other pitfalls (e.g., problems with inappropriate referral, urgency of the clinical situation not appreciated). The authors suggest that these findings can inform education and quality improvement efforts to anticipate and prevent future errors.