Confirmation bias is one of the most prevalent cognitive biases, characterized by the tendency to search for, interpret, and recall only information that confirms one’s own beliefs, while disregarding or minimizing contradictory evidence. This chapter illustrates the phenomenon through the case of arthroscopy in the treatment of knee osteoarthritis, a procedure that was globally adopted for decades despite lacking pathophysiological rationale and evidence of efficacy. Based on isolated clinical experiences, optimistic interpretations, and uncontrolled case series, the illusion of benefit from the procedure was constructed. The randomized, placebo-controlled clinical trial conducted by Moseley and colleagues demonstrated that arthroscopy offered no advantage over sham surgery, revealing the magnitude of the collective error. The chapter reviews the definition, psychological underpinnings, and history of confirmation bias, and analyzes its impact on clinical decision-making, interpretation of literature, adoption of technologies, and resistance to change. Finally, practical strategies to mitigate this bias are discussed, including the deliberate search for contradictory information, intellectual humility, critical thinking, and the use of tools such as pre-mortem analysis.