The Mandela effect is the idea that people have memories that don't match reality. For example, some remember Nelson Mandela dying in prison when he actually became a political leader. This belief is so strong that people think reality has changed. It's like misremembering Darth Vader saying 'Luke, I am your father' instead of 'No, I am your father.' Rather than accepting their fallible memory, people conclude that reality must be wrong. A popular example is Stauffer's Stovetop Stuffing, which some recall but doesn't actually exist.