At the MLB trade deadline, there are teams that get nervous; they do not know if they are buyers or sellers. Others think that this is their one chance to put themselves in a position to win a World Series.
As a result of this, teams make rash decisions. Sometimes, they are willing to give up top prospects for players that do not deliver. There are other times when scouts don't do a great job evaluating the players that their teams eventually trade for.
Almost every single season, it seems inevitable that there is a bad trade made. It may not haunt the team that season, but it comes back to bite it in the future.