Revealing mistake: At the end of the decisive game, the bad guy hits a deep foul ball. During that hit, they show the ball against a black background. If you look very closely at the bottom of the screen, you can see a hand throwing the ball.
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.
Continuity mistake: In the big game the Cubs' manager says to Chet, "You gave me six great innings." But somehow, Henry pitches four innings when there are no extra innings in the game. How unusual.
Continuity mistake: In the final match of the season, the time shown on the scoreboard clock remains the same throughout.
Other mistake: In the last game, Chet pitches six innings ("You gave me six great innings!"), then Henry takes over pitching, meaning he'd be pitching in the 7th inning. Sounds good, until we fast forward to the 9th, with Heddo batting with two outs... Assuming Henry was perfect, Heddo would be the 9th batter Henry faced, meaning he would've been Chet's last out in the 6th, which of course he wasn't (the Chet play at the plate ended the 6th and he wasn't facing Heddo.) Also, Heddo was seen walking up to the plate after the second out of the inning (the rundown at second base), with Martinella saying something like "holy goosebumps, it's Heddo" as he approached. Heddo would've been on deck, so it wasn't a surprise, plus he would've already been at the plate prior to the rundown play.
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.
Visible crew/equipment: When Henry's mom punches Jack, watch as he rolls down the steps: it is not Jack but a stunt double.
Continuity mistake: When one of the batters is hit, watch closely and the ball is tossed from above.
Continuity mistake: When Henry, his mom and Chet are in the airport, the scene shows Henry and Chet just coming out of the gate and they have no bags in their hands. They start running away from the media with Henry's mom across the skywalk, then it shows them exiting the airport still running. Both Chet and Henry have bags in their hands.
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...
Continuity mistake: In the scene where Henry, his mom and friends are going down the escalator from getting Henry's cast off, Henry's Mom is on one side and the boys are on the other. The side that Henry's Mom is on is going down, but the side that the Boy's are on is going up. They are all standing still for a minute, but they stay side by side while one escalator is going up and the other is going down.
Other mistake: When Henry gets his cast put on, there is a brace going from his upper arm to his waist that is molded in place with the cast. After he leaves the doctor's office, it has suddenly disappeared.
Character mistake: When Fish is offering Jack the contract to sell Henry to the Yankees, and later when Jack insults Henry's father, they both refer to Jack as Henry's manager. But Martinella is Henry's manager; Jack is Henry's agent. While the terms can be used interchangeably in other businesses, such as music, in baseball they are two separate titles with separate job descriptions.
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.