Outcome bias is a cognitive distortion that leads to evaluating the quality of a decision solely based on its outcome, without considering the process, the information available, and the conditions under which the decision was made. This chapter illustrates the bias through the controversial Shead vs Hooley case, in which a surgeon was found guilty of medical negligence despite having correctly indicated and performed the standard surgical treatment for a duodenal ulcer. The subsequent complication, gastroparesis, and the devastating outcome following a second surgery performed by another specialist led to a retrospective assessment based exclusively on the final result. The chapter reviews the definition, psychological and historical foundations of outcome bias, from the pioneering experiments of Baron and Hershey to later studies showing how even healthcare professionals tend to judge more harshly when outcomes are adverse. It also explores its impact on medical and surgical practice, including the misjudgment of treatments, blind repetition of previous successes, resistance to change, the creation of work environments focused solely on results, and the undervaluation of critical process analysis. Finally, strategies to mitigate this bias are presented, including structured decision-making methods, documentation of clinical reasoning, counterfactual analysis, pre-assessment of criteria, and seeking independent opinions.