Factual error: For a baseball movie, you would think they would have known the rules of the game better. There is a scene in which Henry uses the "hidden ball trick" to pick a runner off of first base. He stands on the rubber on the mound with only the rosin bag in his hand while the first basemen holds the ball. It is a violation for him to stand on the rubber or straddle it without the ball in his hand and is called a "balk". Technically the runner should have been awarded second base.
Factual error: At the first Cubs home game of the season, you can see that the ivy on the back wall of Wrigley Field is green. The ivy is never green like that during the first two months, at least, of the season.
Factual error: At the end of the season the Cubs end the year tied with the Mets for the Division. This is correct since the MLB didn't switch to 3 divisions until '94 (when there were no playoffs that year due to the player strike). In the 1 game playoff with the Mets John Candy gets excited and declares the winner will go to the World Series. After the game the Coach tells Chet "I'm saving you for the playoffs". If Candy was right about the game then the coach is a complete idiot for not knowing that they are already IN the playoffs. Now they couldn't have been playing in the Division Series (the 5 game first round series) because that didn't exist until '95(would have started in '94 if not for the strike). So either the game was meant to be game 7 of the LCS (League Championship Series), which isn't possible because at the time the Cubs and Mets were in the same division and before '95 (once again due to the strike) 2 teams from the same division couldn't make the playoffs together, or this was a one game playoff to decide the division winner and then the winner would go on to the LCS, which makes Candy wrong. Either way this isn't explained very well at all and confuses many.
Factual error: During one of the games, a batter swings and makes contact. In the next shot (of the Chicago pitcher), the second-base umpire can be seen in the background making the "home run" signal (finger pointed upwards, moving in a circular motion). The next shot, from above, shows the ball bouncing into the outfield for a single. We all hear about how the umpires should get glasses, but first, this one should get a nice ol' pair of eyes...
Factual error: When the Cubs go to play the Dodgers, they show an overhead shot of Dodger Stadium. But during the game, they are not even using Dodger Stadium; they are using the Chicago White Sox Stadium. Dodger Stadium doesn't have a see-through fence in the outfield as it was in the movie, and the sun is in the wrong position for the real stadium. Dodger Stadium also doesn't have a blue hitter's background. It is black.
Factual error: When the game is being played against the Dodgers, the sun is in the outfield and putting shadows facing towards home plate and the batter which is illegal in the rules of baseball. The sun can not be in the batter's eyes. This same mistake has happened in other baseball movies too.
Factual error: The application of the hidden ball trick by the Cubs at the end is illegal. MLB rule 8.05i states a balk is awarded if "The pitcher, without having the ball, stands on or astride the pitcher's plate." Also play doesn't resume after a time-out until the pitcher is on the rubber. AND the only way to pull the hidden-ball trick is if the pitcher is off the mound entirely (check out any of the MLB examples over the last years to see how to legally do it). The rules are written specifically to prevent this type of play. As far as any claim that Henry did not call an "official" time-out, the vast majority of MLB rules regarding timeouts don't require a vocal request. Hitters raising one hand, pitching coaches or managers simply setting foot on the field, all signal timeouts. Calling the entire team in to the pitching mound indicated a cessation in play, which means play cannot resume until Henry toes the rubber and assumes a pitching stance (which he does). Since he transferred the ball to the first baseman while play was halted it is a balk.
Factual error: It is announced that Henry's first game is on August 14. But in the next scene his friends are seen in a crowded school cafeteria. It's summer which means school wouldn't be in session. If it's summer school then there wouldn't be that many kids in the cafeteria and I doubt the girl Henry likes would be attending summer school.