Milwaukee Bucks
Biggest mistake: Not reshuffling the supporting cast
The Milwaukee Bucks are coming off back-to-back disappointing showings in the NBA Playoffs in which they were upset in the first round of the postseason. Heading into the offseason, the expectation was that the Bucks were going to reshuffle their supporting cast a bit - or at least attempt to. So far, Milwaukee hasn't been able to do that much - aside from adding Gary Trent Jr. in free agency. Even then, there's no guarantee he'll be able to make much of a difference for the team.
Something could still change between now and the start of the season, but it does seem as if Milwaukee is going to start the new campaign with essentially the same rotation as it ended with.
Considering that the Bucks needed some sort of change to help them jump-start themselves, their overall inactivity this offseason could end up being one of the bigger mistakes across the league.