As we well know, the play that ended the sixth inning of today's Giants-Dodgers game, on which a run scored, was the result of a bad call, but more interestingly, it was that rarest of baseball events: a situation in which a fourth out would be possible and plausible.

The situation is described on the Wikipedia entry for fourth out. If Pablo Sandoval had, after tagging out the runner, thrown to first and beaten the batter-runner (or if the umpire had made the right call), then the run would not have counted. Although I have not seen the play in question, I assume that this would not be possible. Nonetheless, it is an interesting thought experiment.