The misinformation effect is a cognitive bias that alters memories by incorporating new, incorrect, or incomplete information acquired after the original event, thereby modifying memory and even creating false recollections. This chapter illustrates the phenomenon through the history of gastric cooling and freezing, a treatment proposed by Owen H. Wangensteen for peptic ulcer disease and upper gastrointestinal bleeding. Based on promising preliminary results, the technique quickly gained popularity in the United States, Europe, and Latin America, expanding to increasingly broad indications despite a lack of solid evidence and the early emergence of adverse reports. The chapter reviews the psychological foundations of the misinformation effect, from Bartlett’s work on the reconstructive nature of memory to the experiments of Loftus and Palmer, which demonstrated how subsequent information can distort previously consolidated memories. It also analyzes the factors that promote the spread of misinformation (source authority, anxiety, persuasive narratives, message aesthetics, temporal distance, and repetition) and its impact on contemporary medicine, including medical influencers, predatory journals, distorted clinical decisions, and errors in surgical board meetings. Finally, strategies to prevent this bias are discussed, such as prebunking, structured debunking, use of reliable sources, objective event recording, and critical thinking.