Harris pitched for the Red Sox from 1991-93 and wanted a clause in his contract that he'd be allowed to do it in a game. The Sox refused. Two years later, while pitching for the Expos, he got that chance.
"A natural right-hander, by 1986 Harris could throw well enough left-handed that he felt he could pitch with either hand in a game, but his team would not allow this keeping him from being a legitimate ambidextrous major league pitcher.
Harris wasn't allowed to throw lefty in a regular season game until September 28, 1995, his penultimate game with the Expos. In the ninth inning, Harris retired Reggie Sanders pitching right-handed, then switched to his left hand for the next two hitters, Hal Morris and Ed Taubensee, who both batted lefty. Harris walked Morris but got Taubensee to ground out. He then went back to his right hand to retire Bret Boone to end the inning.
Harris's glove, which was custom built with an extra thumb so that it could be worn on either hand, is now on display at the National Baseball Hall of Fame."