The Mandela Effect is an effect based on a man named Mandela which death was remembered to be in prison, but other people remember him dying from natural causes and going to his funeral. This is what the theory is about, a lot of people remembering something that happened one way, but it actually happened another way. "What is the theory part?" you may ask, well let me tell you. No one really knows how it happens, some people think that we have passed into an AU (alternate universe) when our original universe got destroyed and therefore these small changes are from the new universe we are in. Some of the years where we have supposedly pass into another universe was the year 2000 and the year 2012 (based on the movie 2012 where everything that COULD go wrong GOES wrong). Some of these examples are in this video from a youtuber that I watch and they have 10 examples and its very mind-blowing.