Corrected entry: In an NBA finals series, the higher ranked team has home court advantage. The first 2 games are played at one arena, then the next 2 are played at the other team's arena. Game 6 was played in Boston meaning they were the lower ranked team. Yet the dramatic game 7 was also played in Boston. This is impossible because at least one of the final 2 games would have to be played in Utah, regardless of who was ranked higher.
Corrected entry: At the time the film was made, the Celtics team sucked, always finishing low in their division but for movie purposes they are a good team because they made it to the NBA finals. When Mike and Jimmy are talking to Larry Bird they comment on the Celtics real-life situation that they used to be good but are not any more. Then Bird replies saying he hates fans like that who support the team when they are winning and don't support them when they are losing. Obviously forgetting about the movie plot where the Celtics are actually supposed to be a good team that wins most of their matches, get to the NBA Finals and only losing in 7 games to the team with the best player in the league.