Continuity: The scar on Billy's pitching hand is missing as he sits on his bed in the hotel room after the game. It is also missing earlier in the movie before we knew to look for it.
Revealing mistakes: In the ninth inning of Billy's perfect game, a foul ball is hit by a Yankee into the stands. The negative is flipped - a fan's shirt has the name "Jeter" spelled backwards along with his uniform number "2".
Continuity: Billy Chapel warms up in the bullpen prior to his final game wearing the 1997 uniform of the Detroit Tigers. However, when he takes the mound, and for the balance of the movie, he's wearing the 1998 Tigers uniform. The difference is a prominent quarter-inch navy blue piping on the front of the jersey (around the collar and parallel to the buttons).
Continuity: When Billy is warming up to start the game, the catcher has a protective arm flap on his right shoulder. When the catcher throws the ball down to second base, the arm flap is missing.
Continuity: At the end of the third inning, the announcer says that Billy strikes out the side and now has five strikeouts. Billy strikes out the second batter and third batter in the first inning, and strikes out the first batter (fourth batter in the lineup) at the start of the second inning. During the eighth inning, it's revealed that the sixth batter in the lineup struck out twice previously, which means he struck out back in the second inning. If Billy struck out the side in the third inning, he should have at least seven strikeouts by the end of the third inning, not five.
Continuity: At the end of the third inning, the last Yankee hitter is walking up the first baseline complaining loudly about a called third strike. We know he is the sixth batter because he bats third in the eighth inning (and if it's in the midst of a perfect game, he's batting sixth in the lineup). At the end of the third inning, it'd be the ninth batter that had just struck out, not the sixth batter.
Audio/visual unsynchronized: During the third inning when Billy throws three pitches, all of which are preceded by the announcer saying 'Strike Three,' the same player's picture can be seen on the screen in deep right center field in the background.
Continuity: When Sam Tuttle comes to bat in the first inning the scoreboard indicates that he has hit 39 homers, but when he bats in the seventh his home run total is 19.
Factual errors: When Gus Sinsky is at bat and gets his double, one shot shows the sun in front of him. With the orientation of Yankee Stadium, the only time the sun could be in that position is in the morning, but the game time is late afternoon/evening.
Factual errors: The game takes place in Yankee Stadium, Billy Chapel pitching for the Detroit Tigers. We see the scoreboard and when Billy Chapel gets three outs, they are recorded in the second row: the visiting team usually tracked in the first row, the second row is for the home team.
Continuity: As the third inning ends the TV network scoreboard graphic indicates that neither team has any hits, runs or errors. Moments later a shot of the Yankee Stadium scoreboard indicates the same thing. However, when the fourth inning starts TV announcer Vin Scully states that the Yankee pitcher has allowed two hits.
Continuity: When Mickey Hart saves the would-be Davis Birch home run in the 8th inning, the initial close-up shot sees the hands of fans vying for the ball, but when shown from distance Hart makes the catch on a part of the wall that is nowhere near any spectators.
Continuity: We hear the fan say "kiss that bitch goodbye, take it over" and waves towards the stands. After Chapel "clears the mechanism" they show the exact same sequence with the fan, only without the sound.
Anachronisms: When Mickey Hart makes the leaping catch, to save the home run, you see an advertisement on the right field wall. Yankee stadium would not have any advertisements there until the 2005 season.
Revealing mistakes: In the beginning of the movie, one of the newspaper's headlines states, "Chapel Pitches Lincoln To Title For Second Straight Year." All of the bold print is about Chapel's high school career. However, all of the print is concerning a game the previous night between Detroit and New York in which Billy Chapel pitched.
Continuity: During the first overhead shot of Yankee Stadium, the left field upper deck is completely empty. Later that section is full from a field shot. During a later overhead shot, that section is empty again.
Incorrectly regarded as goofs: The shirt he has on under his jersey had the "home" white "D", instead of the road colors - orange "D". However, there are no regulations as to what a player wears UNDER his jersey/uniform. Billy might simply have chosen to wear a home t-shirt, for any number of reasons.
Errors made by characters (possibly deliberate errors by the filmmakers): Chapel mentions on the plane that he'll be pitching tomorrow and the radio broadcast mentions on game day that it is the second to last day of the season. It is highly unlikely, if not impossible, that a pair of Major League teams would end the season on a 2 game series.