Diagnostic errors have been described as the next frontier in patient safety. Cognitive biases are common causes for these errors but have remained an elusive target for medical educators. This study describes an educational intervention for internal medicine residents consisting of reflective writing and facilitated small group discussions about personal experiences with diagnostic errors. Participating residents identified at least one cognitive bias and one contextual factor that may have contributed to their error. The most frequently implicated biases were anchoring and availability. Most residents also described a strategy to prevent similar errors in the future. A near miss stemming from an initial diagnostic error is highlighted in an AHRQ WebM&M commentary.