A surprising number of people are confident that a famous children's book series was spelled with an "e". The actual spelling — verified across decades of physical books, photographs of bookstore shelves, and authorial documents — is with an "a". Berenstain. Always Berenstain. This kind of mass false memory is now a category. The Mandela Effect, named for the widespread belief that Nelson Mandela died in prison in the 1980s when in fact he was elected president of South Africa and died in 2013. It is the cleanest example of the phenomenon: thousands of people, with no incentive to lie, sharing a memory of an event that did not happen. The official explanation is reconstructive memory. Brains are not recording devices. They are inference engines. They fill gaps with plausible content. Sometimes whole groups fill the same gap the same way. Or — and this is the version that doesn't quite go away — sometimes the system is overwritten and the patches are imperfect.