Diagnostic errors frequently occur because of cognitive errors on the part of physicians. This study used case vignettes to vividly illustrate one specific cognitive error, the "framing effect," whereby a clinician places undue emphasis on a single (often extraneous) piece of information. Inclusion of a single misleading detail resulted in experienced clinicians making significantly more diagnostic errors. The process of meta-cognition, or "thinking about thinking," is often used to attempt to overcome this and other biases in clinical decision-making. An AHRQ WebM&M perspective explores issues related to cognitive errors in diagnosis.