A major source of clinical errors is inaccurate diagnosis. The authors stress the importance of a thorough work-up in establishing a reliable diagnosis and the need to challenge and correct erroneous diagnoses to avoid inadequate response, reduced adherence to treatment, medication errors, increased healthcare costs, unnecessary hospitalization, and other adverse outcomes. Four case reports are presented to illustrate common sources of psychiatric misdiagnosis: effects of switching several psychotropic agents simultaneously, confounding effects of systemic illness and/or substance abuse, poor communication among clinicians and between clinicians and patients, and excessive reliance on the expertise of specialists. In reviewing the cases, the authors focus on "red flags," such as lack of response to the current medication regimen despite adherence to treatment, that may be helpful in identifying diagnostic errors.