I, one of an elite group of esoteric students, have known about The Mandala Effect for a very long time. Allow me the honor of properly defining The Mandela Effect for you so that it can easily be understood:
The Mandela Effect: A social phenomenon where mundanes are amazed to discover that their memories aren't as reliable as they thought they are, followed by completely ignoring that obvious conclusion, and instead speculating ad nauseum about alternate dimensions or some other useless shit like that.
Make no mistake about it. The Mandela Effect is 100% real. The masturbatory speculation that issues from a Dunning-Kruger when it's discovered is 100% not real. In fact, it's counter-productive.
If you happen to be a member of the aforementioned elite group of esoteric students, then you already know our memory is totally fucked. It's one of the first things you learn during your initiation, better known under it's oriental mystic name PSYCH 101. And this, (for those not privileged enough to encounter this esoteric school) is how initiates learn this simple fact.
You will be asked to draw something extraordinarily common, like say, the face of a penny, and that your drawing should could as accurate as possible, including not only the portrait, but also any slogans, dates, letters, etc. Then, the esoteric master (sometimes called a professor), takes these drawings and pins them up where everyone can compare their work with everyone else's. The results tend to look something like this:
And here's the reference version you'll find in most PSYCH 101 books:
How could so many people draw pennies in so many different ways? I know, it's because we come from alternate dimensions where pennies look like this! (Goddamn, I feel like slapping myself, just for saying that nonsense.) Actually, it's because our memory isn't as accurate or as reliable as we think it is.
That's all there is to it.