I might be getting into semantics here but the Mandela Effect is a real thing -- that being the terminology for a collective false memory -- it's the reason it happens that's debated.
I know human memory isn't very reliable so mistakes, even when they're shared by a large number of people, are to be expected. Berenstain vs. Berenstein would be one such example in my mind. Perhaps the teacher who read those books to us in school pronounced it in such a way that the name had more of an E sound to my ear and I just assumed a different spelling than what was actually on the cover of the book. That seems like a likely enough explanation that could apply to other people as well. Also, I imagine the "stein" suffix is more common than "stain" so it's not surprising to me that many people are making the same mistake.
Eli Whitney, as I already explained, is an example of how people could be remembering something accurately, the issue is that the information was given to them incorrectly in the first place. I've heard that many people a generation or two older than I believed for a long time that the correct spelling of dilemma was dilem
na, and they are so sure they were taught this spelling that they think it's likely they
were taught this spelling as a result of a textbook misprint. That sounds plausible to me.
The Columbine one is the one that leaves me scratching my head. How strange to have had this very detailed memory for well over a decade before realizing it was false, but I can accept that I was wrong and perhaps I'd conflated that memory with something else. To then learn that so many other people made the exact same mistake about the date being 1996 doesn't make me feel better, it frustrates me. Because, why? What happened that made so many of us think Columbine happened in 1996? It's just so random. Trust me, I'm not looking for company to make me feel better about being wrong. I'm okay with being wrong, I'd just rather think I'm dumb all by myself.