One of the mistakes that teams are prone to make when evaluating free agent talents is overrating that player’s performance when he plays them. Among the most prominent examples of this misevaluation is when the Indianapolis Colts signed linebacker Erik Walden several years ago, who had some of his best games against them, but was otherwise a profoundly average player.
Another less concerning habit that teams have is focusing too much on players in whom they were interested when they were coming out of college. It goes without saying that a player will go through a lot of changes between their being drafted and their being acquired by a new team, and sometimes that can be for the worse.