#4 - Trading Reggie Smith to the Dodgers
Trading away a future World Series winning, middle of the order bat who would also go on to win a Gold Glove? Yikes.
Reggie Smith was really good for St. Louis during his three-year stretch, posing a .293/.371/.495 slash line with 50 HR and 199 RBI, but those numbers were dipping during the 1976 season. In came the Dodgers, who gave them a few players that would do relatively nothing for St. Louis.
Smith, on the other hand, would be a 145 wRC+ hitter the final seven seasons of his career, becoming a star in Los Angeles. He went to three All-Star Games during that time, and finished top-4 in MVP voting in 1977 and 1978.
Hard the Cardinals stuck it out with Smtih, he may have become a club legend. Instead, he made noise with the Dodgers and the rest is history.