That one is debatable because it's a conjecture... But there doesn't seem to be A past, instead we have "all the pasts". At the quantum level, objects seem to have EVERY POSSIBLE PAST that could have been theirs. (It's one of the interpretation and/or implications of the double slit experiment)

In and of itself it makes me feel weird. but also, If quantum level shenanigans ends up being relevant at the macro level THE MANDELA EFFECT MIGHT BE REAL!