The Mandela Effect is the phenomenon that occurs when a vast majority of individuals collectively remember an event, item, famous quote, or something of the sort that doesn't exist. This, as you may have already assumed, is quite off putting when realized. There are many instances of the Mandela Effect that are floating around the internet and new ones are found all the time.
The Mandala effect gets its name from one of the most popular instances of the effect—the death of Nelson Mandela. After Mandela's announced death in December of 2013, a number of people came forth claiming that they remembered Mandela dying in the 80s. Some claimed, myself included, that they were taught in school that he died in prison in the 80s and even remembered television broadcasts of his funeral. A peak in a history book or a quick google search will lead you to some pretty mixed results. Had Mandela already died or do we all just have terrible memory? If the latter is true, why do so many of us remember the same event?
At this point you may be wondering 'what causes the Mandela Effect'? As of yet there is no set conclusion on the cause of the phenomenon. The most common belief is that our universe occasionally crosses into alternate realities without us realizing it. This leaves us with similar memories of the same events despite the fact that our history records say something completely different.