Diagnostic error is increasingly recognised as a source of significant morbidity and mortality in medicine. In this article, we will attempt to address several questions relating to clinical decision making; How do we decide on a diagnosis? Why do we so often get it wrong? Can we improve our critical faculties?We begin by describing a clinical vignette in which a medical error occurred and resulted in an adverse outcome for a patient. ⋯ The aim of this article is to increase awareness of the role that cognitive bias and heuristic thinking play in medical decision making. We hope to motivate clinicians to reflect on their own patterns of thinking with an overall aim of improving patient care.