May 20, 2010, Reds at Braves. The Reds blew a huge lead by giving up 7 runs in the bottom of the 9th.
Brooks Conrad's pinch-hit grand slam gave the Braves a 10-9 walk-off victory at Turner Field. The grand slam barely cleared the fence, with the ball deflecting off Laynce Nix's glove.
The Reds went to 23-18 on the year.
If you're a masochist, here's video of the entire game.