Joe Niekro drops an emery board out of his pants on the pitcher's mound
In a 1987 game, Minnesota Twins pitcher Joe Niekro was facing the then-California Angels when the home plate umpire noticed something odd. Seeing that the balls Niekro was pitching seemed scuffed by something abrasive, the umpire had Niekro turn his pockets inside out, and an emery board (a flexible, non-metallic nail file) tumbled out of Niekro's pocket onto the grass. The umpire threw Niekro out of the game and he was later suspended.
Why it was so dumb: Niekro, younger brother of Hall of Famer Phil Niekro, initially claimed that he had the emery board with him to file his fingernails smooth to aid his knuckleball grip. This would be dumb enough—why wouldn't he just file his nails in the dugout and leave the thing there?—but man, Joe. Couldn't you have been a bit more subtle?