Factual error: Under the rules of Major League Baseball, a team owner cannot also serve as the team's manager; it constitutes a conflict of interest.
Other mistake: When they do the SPECIAL play, and the pitcher throws the ball to the first baseman, and the first baseman looks for the ball along with the guard and 2 to 3 players. The ball is really at 2nd base, a trick play, but there is NO base coach.
Continuity mistake: In the scene after the Twins lost their first game while Billy and his grandpa were leaving the field, Billy was walking at his grandpa's right side. Then the scene cuts and shows their back. When the scene shows their front again Billy is walking at his grandpa's left side.
Factual error: In the opening scene Billy says the lead runner is entitled to the base if it is occupied by multiple runners. In fact, the trailing runner has this right and those ahead of him on the same bag would be declared out.