@MistyGreenAndBlue
While Clark Gable clearly did say the famous line it is a fact that in no Sherlock Holmes story does he ever say "elementary my dear Watson"
Although this line may have been used in the films. I'm not sure.
He did indeed never say the actual line "Elementary, my dear Watson" in any of the books. But actually I think most people know this now, just like they do know that Frankenstein was the creator (and, of course, arguably the monster too).
But I don't know if things like this count as the Mandela effect. Most people probably haven't read the relevant book, or if they have, it does make sense that they might not recall every detail, especially if the incorrect line is indeed in a film or TV adaptation, which is likely to be more widely seen, or it's just more memorable as a quotation. After all, if the line was spoken in a film, they're not misremembering it, they just aren't aware it wasn't in the original.
I think it applies more to things like, well, Nelson Mandela. How in the heck do you live a life like that, with all the subsequent accomplishments and news coverage, or live through coverage of something like 9/11, and have so many people have the same false memory? It is really interesting.