Background: Diagnostic error is a significant cause of preventable harm worldwide and diagnostic errors have been identified as a high priority patient safety problem by the World Health Organization. Research shows thatdiagnostic error occurs mainly due to system failures and 'cognitive errors' – that is, failure to synthesise all the available information. There is a worldwide consensus that medical schools and postgraduate training programmes rarely teachthe diagnostic process and related decision making (clinical reasoning) in a way that is explicit, systematic and consistent with what is known from research. Materials and methods: This paper presents a short case report and analyses it from a clinical reasoning perspective – performing a 'cognitive autopsy' of a fatal diagnostic error. Results: Clinicians make cognitive shortcuts through pattern recognition and this is highly accurate most of the time. However, shortcuts sometimes go wrong and these are termed 'cognitive biases'. Cognitive biases are subconscious errors of judgement or perception and common examples include 'anchoring', 'the framing effect', 'search satisficing 'and' confirmation biases. These errors are more likely when clinicians are fatigued or cognitively overloaded, and when systems are not designed to mitigate human errors. Conclusions: There is a vast literature on clinical reasoning, 'human factors', and reflection during decision making that show us how we can reduce diagnostic error in our everyday practice. This paper attempts to highlight some of the key findings in the literature that will hopefully encourage readers to explore the patient safety and clinical reasoning literature for themselves and work together to improve outcomes for patients.