And it cost the Cubs a run, and possibly the game.
Here's the play:
The problem, as Hardball Talk pointed out, is a little-known rule in baseball, rule 7.04(c), which states:
If a fielder, after having made a legal catch, should fall into a stand or among spectators or into the dugout or any other out-of-play area while in possession of the ball after making a legal catch, or fall while in the dugout after making a legal catch, the ball is dead and each runner shall advance one base, without liability to be put out, from his last legally touched base at the time the fielder fell into, or in, such out-of-play area.(highlighting mine)
So by catching the ball while falling out of play, Rizzo allowed the run to score. Had he let the ball drop foul, there still would have been runners on first and third, nobody out, 0-1 count on the batter. So there's still a pretty good chance the Diamondbacks would have scored a run. (To date this season, teams have scored 1.67 runs in innings in which they had runners on first and third with nobody out.) But they might not have. And that one run was the margin of victory in a 3-2 win for Arizona.