C. Emery board
After Joe Niekro's third pitch to the Angel's Brian Downing, suspicious umpires swarmed the mound and required Niekro to empty his pockets. As the Los Angeles Times notes, Niekro – in front of the umpires, 35,000 stadium spectators, and a national TV audience – attempted a slight of hand to casually flick away the emery board in his back pocket. It didn't work. He was ejected from the game. The Twins went on to win 11-3. Niekro was suspended for 10 games but went on to pitch for the Twins in that season's winning World Series.
Here's video of Niekro's slick move along with sportscasting by Ron Futrell, with the classic line that Niekro's knuckle ball was "singing the National Anthem" on the way to the plate.
Niekro insisted he needed to have short fingernails in order to throw his trademark pitch.