#12 - Trading Ken Boyer to the New York Mets
Boyer won the National League MVP in 1964, and after dropping off during the 1965 season, was sent to the Mets in exchange for third baseman Charley Smith and pitcher Al Jackson. Smith was fine at third base for the Cardinals in his only season with the club, while Jackson was incredible in his first year with the club.
Despite going 13-15, Jackson put up a 2.51 ERA in 232.2 innings of work for St. Louis in 1966, and after faltering some the season after, made his return to the Mets in 1968.
While Boyer only had two well-above-average seasons over the last four years of his career, parting with a legend like that is a tough pill to swallow. Jackson's performance makes this trade not a complete fail, but I'm sure they'd like to have ended on a better note with their Red Jacket club member.