Some people automatically assume they are always right, combine this with remembering something wrong and Deja vu and you get the Mandela effect. An example is the actor Emilio Estevez (Charlie Sheen's brother), many people believe he died in the 90's. As proof they site he had a very successful movie career, then disappeared. In actuality he is still alive, just does more directing now. This is from people assuming something, then with the passage of time the belief is reinforced as absolute truth, compounded by the sense they must be right.