10. 1990 - Red Sox - Larry Andersen
This trade holds the honor of being both an impactful midseason trade as well as one of the worst trades of all time. Feeling that they needed help with middle relief to hold off the Blue Jays in 1990, the Boston Red Sox made a late season deal for Larry Andersen. To get Andersen from the Astros, the Red Sox had to part with a prospect named Jeff Bagwell.
Andersen pitched 22 innings for the Red Sox, giving up just three runs and was instrumental in their holding on to win the AL East by two games. However, in Game 1 of of the ALCS against Oakland, Andersen took the loss for the Red Sox. Andersen pitched well in two more innings of relief during the series, but the A's nonetheless swept the Boston.
The Red Sox were not able to keep Andersen as he left for San Diego via free agency in 1991. Worse, the Red Sox had to watch Jeff Bagwell win rookie of the year the very next season and go on to have a great career in Houston.