Diagnostic error continues to be a source of preventable patient harm. The authors undertook a retrospective review of primary care consultations to identify incidence, origin and avoidable harm of missed diagnostic opportunities (MDO). Nearly three-quarters of MDO involved multiple process breakdowns (e.g., history taking, misinterpretation of diagnostic tests, or lack of follow up). Just over one third resulted in moderate to severe avoidable patient harm. Because the majority of MDO involve several contributing factors, interventions, including policy changes, should be multipronged.