and he has a scene about "HEY! I DID EVERYTHING AND I GET NOTHING?!" that is completely missing from my childhood version?
You remember things misspelled because, in most cases, the real spellings are different from how they "should" be spelled and you simply didn't pay attention. E.g., "Cap'n Crunch": It's pronounced "Captain Crunch," it's mentally remembered as sounding like that, so, therefore, you assume it's actually spelled that way without really looking at the cereal box to find out.
The movie quotes: They're misremembered because a) they become popularized through some comedian or talk-show host riffing on it incorrectly, and it's the mis-memory that is remembered, or b) they become generalized ("build it and they will come" is now a line used to encourage someone to do something, so saying "build it and HE will come" makes zero sense to say. Or "Life is like a box of chocolates" is just a way of saying that life is full of surprises. It makes no sense to say "Life WAS like a box of chocolates" to someone to indicate that life can throw some curve balls. In the Gump movie, however, Hanks's character is mentally challenged and speaks in a way that reflects that fact. When you're talking to a friend and trying to indicate that life is full or twists and turns, you are speaking in the present tense and are, presumably, speaking normally. So *you'd* say that life IS like a box of chocolates. Since "life is like a box of chocolates" and "build it and they will come" are the phrases most often heard after a quote has become generalized, people mis-remember "having heard" those lines spoken that way in the original movies as well.
So, you go through a website and see page after page of mis-memories, and then feel as if you can't trust your instincts when, in fact, there's nothing wrong with you or your instincts or your memories. You simply weren't paying enough attention in the first place, and remembered things incorrectly for logical, sane reasons. There is no "parallel universe." The fabric of space-time is just fine. God is in His Heaven and all is right with the world.
--- but it's fun to see these mis-memories all in one place! So I'm glad that the "Mandela Effect" idea is out there for that reason. I just wish people would think more logically about it all and not freak themselves out over it.