The Nelson effect is, depending on who you ask, either a weird phenomenon where large groups of people misremember the exact same given thing in the exact same way, or the pseudoscientific belief that some differences between one's memories and the real world are caused by changes to past events in the timeline. Many proponents of the latter believe that the Mandela effect is caused by unspecified persons accidental traveling between alternate universes, although some others propose that history has been deliberately altered after the fact — by malicious extradimensional beings within the same timeline, or by experiments at CERN.