Retrospective bias is a cognitive distortion whereby events, once they have occurred, seem to have been predictable, obvious, or even inevitable, regardless of the information available prior to the outcome. This chapter uses the historical case of president James A. Garfield’s death to illustrate how this bias shapes the way we judge past decisions, particularly in medical contexts. Although today the errors in his care are considered a classic example of negligence, the physicians acted according to the prevailing knowledge in 1881, at a time when the principles of antisepsis and germ theory were not yet established in the United States. Through analysis of Fischhoff’s original experiment and subsequent studies, the cognitive mechanisms by which known outcomes alter the interpretation of the past are described. The consequences of retrospective bias at both individual and collective levels are also reviewed, including flawed judgments, overconfidence, errors in historical assessment, and distorted medical decision-making. Finally, practical strategies are presented to reduce its impact and promote more objective and mindful analysis in surgical practice and outcome evaluation.