When Lindsey is talking to her mom in the kitchen, there is a plate on the counter with only four cookies on it. In the next shot, when her mother picks up the plate to offer some to her, the plate is full.
When Lindsey gets to her seat, at the game in which Ben is selling his tickets, Johnny Damon is clearly in center field, as it is the top of the 9th, and the Red Sox are on defense. However, when she looks through the binoculars, the first base coach is from the Red Sox, and the first baseman is a member of the Yankees, meaning the Yankees were playing defense, and the Red Sox were batting.
When the ball is fouled off by Miguel Tejada and hits Lindsay, the pitch is a left-handed submarine pitch. During the late news, it is a regular left-handed pitch.
When Lindsey goes to Ben's apartment after returning from her parent's house, they go into his bedroom where Ben gives her the Red Sox jacket. In first few shots of her wearing it, the American flag is visible on the front of her right shoulder. However, for the rest of the scene, the flag is not visible at all.
When Ben and Lindsey are having their BBQ in the park, Ben is holding his cup in his right hand. Change angle on shot, and he is gesturing with his right hand, now empty. Change angle, it's back again.
When Ben goes to his first game, during batting practice they show the second baseman wearing number 14. In 1980 number 14 belonged to Jim Rice. Jim Rice is black, the second baseman is white.
There is a scene in which we find out the Red Sox have scored eight runs in the bottom of the ninth inning to defeat the Yankees 8-7. During the 2004 season, no such game occurred.
When it is revealed that the Yankees officially clinch the division, the "F" indicating the final score goes up from inside the hand-operated scoreboard while an inning is in progress. This is OK. The American League out-of-town scores are changed from behind. The National League out of town scores are changed from the outside and only between innings.
When Ben asks Lindsey to opening day, his ticket says Tuesday April 11. April 11 was a Sunday in 2004. Also, opening day was not on a Tuesday, it was on a Friday.
During game 4 of the ALCS while it is showing Ben, in the background we can hear an announcement for the Yankees' John Olerud. Olerud did not play at all during that game.
When Lindsey is running across the field, it appears that she throws her purse at a security member. Seconds later she runs around Johnny Damon and her purse is back in her hands for the rest of the scene. However, she actually takes Damon's glove and throws that at a security member, not her purse, which correctly stays on her arm during the entire scene.
When Lindsey is looking through the binoculars, she appears to look through them backwards, with the smaller "eyepiece" facing outward. She is actually looking through them correctly. The particular binoculars are a compact style that uses a reverse Porro prism design, which allows the objective (outward) lenses to be closer together. When she has them in her hand, you can see rubber eye cups on the end she holds to her face.
When Ben first meets Lindsey's parents, he hopes to create some familiarity with them over dinner. He talks to her mother who was a teacher and is now a principal because he is also in the education field; but he mentions when Lindsey's dad says he sells golf carts, he says "that gives us nothing to talk about." Actually he was only talking about Lindsey's dad who sells golf carts., not incorrectly talking of both.
While it is believed that blocking tape is visible in the shot at Robin's birthday party where Ben doing the worm, it is actually a floor plug.
When Lindsey gets hit with the foul ball, she is holding a laptop computer. In the replay on Sportscenter the laptop is nowhere to be found.
When Ben and Lindsey attend their first game together, there are numerous dummies and mannequins sprinkled throughout the stands behind them. This implies this scene was not shot during an actual game.
With about ten minutes to go in the film, and during the "attempt to buy the season tickets" scene, if the viewer looks at the top of Fenway crowd scenes, we can see a "dummy board" which fills in the top of the shots. Those "people" at the top of the shots, have been painted in to look like fans sitting up top and in the background.
In the first scene when young Ben walks into Fenway Park, Jim Rice and Dennis Eckersley are clearly their 2004 selves, and look considerably different than they did in 1980.
In the Opening Day scene when they're getting into their seats, look at the upper right hand part of the screen and you'll see the dummies they're using to act as seat fillers.
The Red Sox schedule that Ben has displayed in his classroom shows that the series vs the Yankees, which included the game Ben missed to go to the party with Lindsey, was their final home series of the season. The home game in which Ben and his friends found out the Yankees clinched the division happened after he and Lindsey broke up.
When Lindsey tells Ben she has the chance to go to Paris and take someone with her she says "and I am taking vous". People who are familiar with each other would not use the more formal "vous" but the more familiar "toi".
Right before the scene where Lindsay gets hit with the ball, the legend on the screen says it's July. The ball Miguel Tejada of the Orioles hits into the stands, which hits Lindsay, is thrown by Mike Myers of the Red Sox. Mike Myers wasn't picked up by the Red Sox until August 6, 2004.
In a scene set in 1980, Uncle Carl's car shows a modern Massachusetts emissions sticker on the windshield.
The banners hanging outside Fenway with dates of World Championships on them were not there in 1980.
During the party scene we hear the song "O Babe, What Would You Say" By Norman 'Hurricane' Smith. The saxophone player at the party seems to be playing but it is the sax solo from the original song that we hear.
Reflected in Troy's sunglasses at the game where Ben is explaining how the team was a second family.
The narrator says that at Ben's first game at Fenway Park in 1980, Dwight Evans hit two home runs. Evans did have a two-homer game in 1980, but in Minnesota, not Boston.
Lindsey's office is first shown to be in the John Hancock Tower (big blue glass building), but towards the end of the film it's shown to be in the Prudential Tower (which has "GO SOX" lit up in the windows).
When Lindsay introduces Andrew Wilson's character to her coworker, she says his name is Patrick Lyons but the movie credits list him as Grant Wade. Nowhere in the movie is the name Grant Wade mentioned. He is repeatedly introduced as Patrick.