How unlikely was it? Let us count the ways.
While Mariano Rivera is the best closer in baseball history, he's a bit past his prime. The best closer in baseball at present is Craig Kimbrel of the Braves. He came into Tuesday having closed down 47 of 50 save chances with a 1.04 ERA, 0.86 WHIP and 89 strikeouts in 60 2/3 innings.
So Kimbrel is ridiculously dominant. We know that. He rarely coughs up saves. We know that. But how about this one: Kimbrel entered Tuesday having never in his big-league career allowing three runs in one outing. Yes, one run was unearned, but he had never previously given up three runs in an outing, regardless of if they were earned or not.
Speaking of a run being unearned, this happened because Andrelton Simmons committed an error -- as those who watched the video saw. Simmons entered the game with a .984 fielding percentage, having only committed 11 errors in 690 chances this season at short.
Not only that, but Simmons is having a historically great defensive season. I've covered it before, but since then Simmons has set a record for defensive runs saved with ease and also has the highest-ever defensive WAR on baseball-reference.com -- topping the likes of Brooks Robinson and Ozzie Smith toward the top of the leaderboard.
So teaming up the Nationals scoring three runs on Craig Kimbrel in one inning to win the game with an Andrelton Simmons botched defensive play opening the door makes this the odds-on favorite for the most unlikely win of the season.