#6 - Trading away Jerry Reuss
Jerry Reuss pitched in 57 games for the Cardinals during his age 20-22 seasons, amassing a 22-22 record with a 4.43 ERA in 345.1 innings pitched. Trading him away would not have shocked anyone, but trading him turned out to be a big mistake.
Reuss ended up pitching for 22 years in the league, posting a 220-191 record with a 3.64 ERA in 3669.2 innings for the Dodgers, Pirates, Astros, White Sox, Angels, Reds, and Brewers, along with his debut team, the Cardinals.
Not only was trading away that kind of longevity a mistake, but he also had two All-Star seasons mixed in there, including top 20 MVP finishes twice and second in Cy Young voting in 1980.