There was a book I read years ago in which someone invents a device that lets them change history*. It's not exactly a time-machine, in the usual sense, because it doesn't let the character travel back through time. Rather, the invention allows them to kind of reach back through time and meddle with events, while remaining in the present. But after they mess around with history, they realize that everything is worse, and they spend the rest of the book trying to undo what they did. Of course the concept of alternate, parallel universes has really been in vogue lately (although it's not a new idea**).
Do you ever wonder if we're living in the weird, alternate-history universe where something got screwed up and events unfolded in some totally bizarre way they shouldn't have? Or maybe there was that odd, 1-in-a-million chance of something strange happening, but with a million chances, somebody had to get the 1, and it was us? Like, if you could look at the nearest 999,999 universes, you'd think 'hey, how come they all have _____, and we don't?'
* Had to look it up. It was Making History (1996), by Stephen Fry. I remember finding it so funny, I was worried people around me would think I was having some kind of fit.
** I think the first 'multiverse' movie might have been It's a Wonderful Life (1946).