Illusory correlation is a cognitive bias that leads individuals to perceive nonexistent causal relationships between two events that occur simultaneously or sequentially. This chapter illustrates the phenomenon through the historical case of bilateral internal mammary artery ligation for the treatment of angina pectoris, a procedure that persisted for over two decades despite lacking physiological basis or clinical evidence. The combination of isolated observations, biased interpretations, professional enthusiasm, and media dissemination fostered the false belief in a causal link between intervention and symptom improvement. The chapter reviews the definition of illusory correlation, its conceptual foundations, and the classic studies by Chapman, Hamilton, and Gifford, as well as contemporary developments that explain why humans tend to perceive nonexistent patterns. The impact of this bias is analyzed at the personal, collective, and clinical levels, including the formation of stereotypes, adoption of irrational behaviors, proliferation of conspiracy theories, and medical reasoning errors. Finally, strategies to mitigate its effects are discussed, such as critical thinking, structured data presentation, exposure to diverse perspectives, and explicit acknowledgment of our vulnerability to bias.