A false memory is a psychological phenomenon where a person recalls something that did not happen or happened differently from the way it actually happened. In 2010, this shared false memory phenomenon was dubbed the "Mandela Effect" by self-described "paranormal consultant" Fiona Broome, in reference to a false memory she reported of the death of South African leader Nelson Mandela in the 1980s (who actually lived until December 2013), which she claimed was shared by "perhaps thousands" of other people.

UPD: Title changed.