Probably the Piazza/Poop Game. It was September 2004, Phillies leading the Mets 4-3 with 2 outs in the top of the 9th. Mike Piazza's at the dish, and he crushes a hanging slider from Jose Mesa that clears the brick facade out in center field. Then as he's rounding second base, pumping his arms up and down, a very noticeable turd falls down and out of his pant legs. Piazza pretends that nothing happened and continues rounding the bases, but Joe West (the home plate ump) walks out and picks up the piece of poop. He squishes it around in his hand, smells it, and then points at Piazza (who is walking back to the dugout and high-fiving teammates) and calls him out, ending the ballgame.
Of course, Piazza goes crazy and rushes out to West, getting in his face and yelling "It's not my fault!!! I'm a catcher in my personal life too you fucking homophobe!!!" (it was a bad night for him to be mic'd up) but Cowboy Joe was having none of it. It was a historic game to go to and I'll never forget it.