Dallas Mavericks
Biggest mistake: Letting go of Derrick Jones Jr.
Riding the high of making the NBA Finals, even though they fell quite short against the Boston Celtics, it was impressive to see the Dallas Mavericks continue to push the envelope at the start of the offseason. The Mavs could've been happy with their run to the NBA Finals and sat out the offseason. However, they didn't and continued to work to improve their roster. While adding Klay Thompson will prove to be a huge win for the team, having to let go of Derrick Jones Jr. in the process could hurt the team in other ways that aren't visibly evident.
The athleticism he brought on the wing may not be able to be matched by the additions the Mavs made this summer. While his production may be filled by the likes of Naji Marshall and Quentin Grimes, two offseason additions, I'm not sure if either of those players will bring the energy as DJJ continuously did throughout their playoff run.
The Mavs will be relatively fine, but there's at least a small chance that they look back at letting DJJ walk as a big mistake that hurt their team chemistry.