The Mandela Effect was briefly covered by Eddie and Chief on a Dogwalk a little while back.
Its theory is based on an unusual phenomenon where a large group of people remember something differently than how it occurred. Conspiracy theorists believe this is proof of an alternate universe, while many doctors use it as an illustration of how imperfect memory can be sometimes.
Either way it's really bizarre shit. Especially when you get to talking about The Berenstein Bears and Monopoly Man. You can tell me all you want that they're not how I remember and I'll tell you you are a liar and/or in on the conspiracy.
When you start getting into the theory of it relating to parallel universes is when it gets really hairy.
h/t to @paulienutwall for the suggestion. Keep them coming. And keep it classy. No butt stuff