Multiverse theory says that there’s an untold quantity of universes out there, and each one might be different based on a single choice that one person made. There’s a universe out there where, for example, I live in outer space, because the Library of Alexandria never burned down and the advancement of human knowledge wasn’t set back by centuries. If you had the opportunity to fix this, would you? That’s a pretty huge reset, so it’s hard to begin to imagine the ramifications of doing so. What about, say, going back three hours to create a timeline in which your friend’s pet isn’t murdered and your godfather doesn’t receive the Dementor’s Kiss? This presumably creates a better future, as it results in Sirius living, which gives Harry access to Grimmauld Place, which gives him access to Kreacher and the story of Regulus, etc. As an author, resetting the timeline is something you should only do in the worst possible circumstances, and even then, it’s hard to argue that retconning a given set of circumstances that are bad for your characters is worth retconning an entire universe for.
I learned while writing this post that ‘retcon’ is a portmanteau of ‘retroactive continuity’.
Lady Geek Girl and Friends 