The alternative history novel “The Man in the High Castle” features a “novel within a novel”. While the actual book is about Nazis winning WWII, the in-book novel is about an alternate universe in which the Nazis lose the war.
“The Mandela Effect” is a term coined to describe the phenomenon of large numbers of people remembering something one way, and others remembering it another way. The theory is that both groups of people are correct, but one group came from one timeline or reality and another group came from another timeline or reality. Among other things, they remember Mandela dying in the 80s.