Other mistake: In the championship game, the Yankees batter hits a foul ball when they score their winning runs.
Deliberate mistake: In the championship game, when number three on the other team hits a home run, he never touches home plate. Home plate is where all of the players are crowded around (primarily the ones to the left of the scene). The player makes a "jump stop" at the batter's box. The Bears could have waited 'til the player went back to the dugout; the catcher, with the new ball in play, would have stepped on home plate, and the home run never would have counted.
Deliberate mistake: When Buttermaker goes to the trunk of his car to get the new team jerseys, he opens a box sitting on top of a pile of stuff. There is no way that this box would have fit in the trunk with all of this stuff and closed properly. The front of the car was loaded with Buttermaker and some of his team, so it couldn't have fit on the seats, either.
Continuity mistake: In the last game when Justin is finally allowed to hit the ball, the pitcher makes two throws but the score has 2 strikes and 1 ball.