Cincinnati trades Frank Robinson for Milt Pappas.
Frank Robinson hit the ground running for the Cincinnati Reds in 1956, winning Rookie of the Year. He went on to have a tremendously productive decade, putting up power hitting numbers comparable with Hank Aaron and Willie Mays (of whom you may have heard). Robinson won the MVP in 1961, when the Reds made the World Series. And he enjoyed a typically productive year in 1965, during which, in a sign of the fearful respect with which he was regarded by National League pitchers, he was among the league leaders in being walked intentionally (he'd led the league the previous four years.)
But where the rest of the National League saw one of the scariest sluggers in the business, Robinson's general manager saw a decrepit, broken man on the verge of death. He was, after, all thirty years old (GASP!) And so before the start of the 1966 season, Robinson was traded to the Baltimore Orioles for pitcher Milt Pappas, who didn't suck, but that was about all you could say for the guy.
Why the trade was so dumb: Milt Pappas lasted two and change seasons with Cincinnati, most of which time he was battling injuries. Frank Robinson, on the other hand, went on to win the Triple Crown and a World Series (the first of four in six years) in the first season he was in Baltimore, giving hope to all grizzled, hunched, thirty-year-old shadows of their former selves the world over.