2011 Boston Red Sox
The Red Sox led not just the wild card, not just the division but the entire American League going into September. Boston went 7-20 the rest of the way, seeing the hated Yankees pass them for the East lead and, on the final day of the season, the Rays complete their comeback from nine games down to snatch the wild card. On the final night, the Red Sox lost on an Orioles walk-off in the ninth inning, and the Rays, down 7-0 in the eighth, walked off on an Evan Longoria home run in the 12th to beat the Yankees. The two games finished within minutes of each other. The Red Sox's September collapse, squandering a nine-game lead, is the greatest in MLB history. Only one team, the Twins (6-20), was worse over the final month.