The Mandela Effect refers to a curious phenomena where large numbers of people remember a particular event in a way that conflicts with how it actually occurred according to available evidence. To be perfectly clear, this is not "Funny, I remember it like ..." instead this is where people have clear, precise, detailed and usually fairly vivid memories of an event that either never happened or that happened very differently from recorded history.
The term was coined by author and paranormal investigator Fiona Broome. When Broome attended Dragon Con in 2006 she was shocked to discover that Nelson Mandela was alive despite having numerous, explicit memories of his death in the eighties, complete with news pieces on the effect on South Africa and interviews of his widow. When she mentioned her false memories to other con goers she was again shocked to discover that several people around her shared her recollection of the passing. Not one to miss an opportunity to study weirdness, she wrote a blog post detailing her experience and asking other to share any memories that conflict with reality.
Now if you're a psychologist or a sociologist you'd chalk this up to some quirk of human memory. On the other hand, if you're a paranormal investigator you'd speculate that history is changing. Throw in some dubious connections to the many worlds interpretation of quantum mechanics and you've got a working pseudoscience theory.
Maybe that's all it is but it's startlingly common. Some major examples included:
Remembering the Berenstain Bears as the Berenstein Bears.
Remembering Darth Vader's line as "Luke, I am your father" rather than the correct "No, I am your father."
Curious George having a tail. This one is unique in that many people remember it both ways at once.
Islands such as Japan, New Zealand, and places such as Wales changing location.*
A mid season Star Trek: Voyager episode where Chakotay died only to return without explanation a few weeks later.
Billy Graham's funeral being televised in 2009.
These are just the highlights. For more on the Mandela Effect check out its website here or its subreddit here. If you've experienced a clear conflict between your memory and history message me the story and I'll copy/paste it into this write up.
RUST IS FOR THE WEAK