Boston Celtics
Biggest mistake: Not adding more depth to the frontcourt
It's hard to look back at anything the Boston Celtics have done over the past calendar year and declare anything as a disappointment or a mistake. Having dominated their way through the Eastern Conference, and then in the NBA Finals against the Dallas Mavericks, the Celtics are in a position where they're in a great place to defend their title. Even though they haven't been all that active so far this offseason, the Celtics have done exactly what they needed to do in locking up the foundation for their future.
However, if we are to nitpick the team, there is an argument to be made that the Celtics needed to add a bit more depth to the frontcourt heading into the start of the season. Under the impression that Kristaps Porziņgis is going to miss, at least part of the start of the season, it would bode well for the Celtics to add a bit more depth at the power forward or center position.
As they head into the final stretch of the offseason, that's something that almost has to be done. If it isn't, it's going to be looked at as a huge oversight on the part of the front office.