The bandwagon effect is a cognitive bias that leads individuals to adopt beliefs, decisions, or behaviors simply because they are perceived as popular or widely accepted, without a critical analysis of the available evidence. This chapter illustrates the phenomenon through the history of the rise and fall of the adjustable gastric band, a procedure that, despite limited initial evidence and a lack of robust long-term outcomes, quickly became the most commonly performed bariatric surgery worldwide. The combination of a growing need to address the obesity epidemic, the appeal of a reversible and minimally invasive procedure, enthusiastic early reports, the influence of opinion leaders, media endorsement, and social pressure led to widespread adoption that far outpaced any scientific support. The chapter reviews the psychological foundations of the bandwagon effect, its history from 19th-century bandwagons to the conformity experiments of Sherif and Asch, and its impact on personal, collective, and medical decision-making. Contemporary examples of premature adoption of surgical technologies, overuse of diagnostic studies, and prescription of fashionable treatments are also analyzed. Finally, strategies to mitigate this bias are offered, such as critical thinking, deliberate search for robust evidence, consideration of minority alternatives, and explicit recognition of social pressures in decision-making.