The Mandela Effect is where people have false memories of past events, and decide that there's been a jump to an alternative universe where history is different.

The name comes from the first reported instance of this effect being noticed, where several people at a convention found that they had the same false memory. Blogger Fiona Broome wrote:

I thought Nelson Mandela died in prison. I thought I remembered it clearly, complete with news clips of his funeral, the mourning in South Africa, some rioting in cities, and the heartfelt speech by his widow.

Then, I found out he was still alive.

A physicist had a similar experience with the name of the Berenstain bears, which led him to postulate:

I propose that the universe is a 4-dimensional complex manifold. That means I propose the 3 space dimensions and the 1 time dimensions are actually in themselves complex, meaning they take values of the form a+ib, part "real" and part "imaginary". Within this 4D manifold, there are sixteen hexadectants (like quadrants, but 16 of them), corresponding to whether we consider only the real or imaginary part of each of the four dimensions. In our particular hexadectant, the three space dimensions are real, and the time dimension is imaginary.

Rather than accept that their memory is flawed, which is the more simple, logical explanation, there are a whole host of ideas about how multiple universes can account for the problem.

Some people "slide" between universes - either their body, memories or consciousness.

A recent event that has been picked up as a potential Madela Effect event is Leonardo DiCaprio's winning of his first Oscar. Some people swear that he won the best actor Oscar for Titanic back in the '90s.

There's even an alternative universe suggested for the location of New Zealand, although nobody seems to agree on whether we used to be East of Australia, or North-East, or even North-West.