Michael Shermer described the Mandella Effect for eSkeptic in 2015:

The first student TED talk was by Taryn Honeysett on something called “The Mandela Effect,” of which I was unfamiliar. The name comes from the mistaken belief that the great statesman and civil rights activist Nelson Mandela (1918–2013) died while in prison in the 1980s, and it is characterized by a group of people who all misremember something in a similar manner.

The effect gained a cultural toehold in an Internet forum discussion over the proper spelling of a popular children’s book and television series called The Berenstain Bears, when a number of people insisted the correct spelling was Berenstein.

It’s a popular topic on the Overnightscape Underground. I can’t say I ever thought Nelson Mandella died in prison; he had such a huge effect on South Africa and the course of global history. And they were always the Berenstain Bears to me.

The closet I’ve come to this effect is the pronunciation of Project. Before our family moved to Singapore from Australia, I felt as though everyone pronounced it the same was as Progress, with the first syllable rhyming with grow. Since coming back, everyone I meet in Australia rhymes it the American way with frog. It’s surreal.

John Gruber linked to an old article by Major Keary in PC Update:

American English relies on the sound of a word to determine where it should be broken. The English first turn to etymology, and then to sound. That may seem to be a very minor difference, but there can be marked variations in pronunciation which produce quite different points of division. Progress is an example. In American English the first syllable rhymes with frog and is the accented part of the word. Progress as a noun is pronounced in the U.K. and Australia with equal emphasis on both syllables and the first rhymes with throw. The result is that prog/ress is the correct U.S. point of division, but pro/geess [sic] is correct British usage.

So the way I’ve always said project would be pro-ject. But the American way, and the way all my friends and colleagues in Australia now say it, it’s proj-ect. Even though Australians (and the Brits) haven’t changed the way they pronounce progress to match. It makes no sense.

Does that count as a Mandella Effect?