Memphis Grizzlies: Allowing the Celtics acquire Kristaps Porzingis
Seeing how well Kristaps Porzingis is playing for the Celtics, if any team had helped bring the big man to Boston, they'd all probably prefer to send that back. The Grizzlies helped Boston in the trade that sent Tyus Jones to Washington, Marcus Smart to Memphis, and KP to Boston.
In hindsight, the fit between Porzingis and the Celtics is perfect and they are currently the best team in the NBA. Memphis, meanwhile, started the first 25 games of the season at 6-19. I'm sure they'd reconsider this move in hindsight, which is why this move should be deeply regretted.
Miami Heat: Not running it back with their undrafted shooters
If you watched the Eastern Conference Finals last year, the announcers could not and would not stop talking about the amount of undrafted free agents the Miami Heat were playing, all of whom playing well.
Max Strus and Gabe Vincent landed on the Cavs and Lakers, respectively, via free agency. Although Miami is not playing poorly this season, why not run it back with your guys and provide this team with some depth? They could use another wing and they could definitely use some guard depth, especially at the point guard position.