Marvel Comics used to have (may still for all I know) a series by that name. The basic gist is that they would take some of their most common stories (frequently involving the origins of a particular character) and tweak it, some times in a very small way. The point was to show how a very small difference could drastically change events.
I was thinking about this while watching the History Channel. They were discussing the destruction of Archimides's works were destroyed with the loss of the Great Library in Alexandria. It raised the obvious question, "What if the Great Library had survived?" When you realize all the items we KNOW were lost, to say nothing of those treasures we have no clue about, it becomes a really staggering question.