Factual error: In the opening day clips, they show 49, Tim Wakefield, starting. The starting pitcher that day was 38 Curt Schilling (Wakefield never played that day). Furthermore, the clips show them playing against the Texas Rangers; opening day 2004 was against the Toronto Blue Jays.
Factual error: In the scene in the park where Ben does his fake proposal bit and asks Lindsey if she'll go to opening day with him, the ticket is for Tuesday April 11th. It's obviously fake for two reasons. First, the Red Sox opening day in 2004 was April 9th, not the 11th. Second, April 11th, 2004, was a Sunday, not a Tuesday.
Continuity mistake: When they show a highlight from ESPN when Drew Barrymore gets hit with a foul ball, Miguel Tejada is supposed to be at bat. The clip shows some random guy taking a cut and fouling the ball off. Not a big deal I guess, but I noticed it quickly.
Continuity mistake: When Drew Barrymore is talking to her mom after her dad has just walked out of the kitchen, you see the teapot facing towards the right of the screen. However, in the next shot of it, it is facing the other way, towards the left.
Continuity mistake: When Lindsey is at her parent's house during spring break, they are discussing her father's hair. In the shot when her mother opens the fridge there are only 5 cookies on the plate, but when the shot changes and Lindsey is grabbing some cookies there is a whole plate of them. And the honey pot is missing too.
Factual error: Towards the end of the movie, when they go to game four of the ALCS the announcer at the stadium announces: "Now batting John Olerud" but John Olerud didn't play in game four of the ALCS because he hit his foot with his bat during game 3 of the ALCS.
Visible crew/equipment: When Jimmy Fallon and Drew Barrymore are taking their seats after the guy yells peanuts and crackerjacks, if you look carefully you can see some of the seats behind are actually filled with mannequins dressed with clothing. Some of them don't even have arms.
Other mistake: When Ben gets home from the costume birthday party and laying in bed he puts the TV on when he finds out the Sox won the game. The alarm clock on the nightstand reads 10:38 when he first puts the TV on - then a couple of TV clips later it is 11:27.
Factual error: When Ben and Lindsey have their fight over going to Paris, Ben says he can't because the Mariners are coming to town (his schedule on the chalkboard says 9th-12th). The Sox did play the Mariners on the 9th-12th, but it was in Seattle, not Boston. Also, when Lindsey says "Pedro's pitching Friday", Ben answers, "Saturday, Schilling's on Friday". Schilling did pitch that Friday, but Bronson Arroyo was the Boston starter on Saturday. Pedro Martinez didn't pitch again until the 14th, against the Devil Rays.
Continuity mistake: When Lindsey takes off her shoes and starts running to Ben, the shoes she took off are gone from the field in the following shots.
Continuity mistake: When one of Lindsey's friends gets hit during boxing practice at the gym, the size of the sweat marks on her shirt changes between shots.
Plot hole: After Lindsey is seen getting an MRI scan, Ben is in her kitchen and receives a phone call from Troy saying that Lindsey is on TV. But when Ben turns the TV on, the news anchor is just introducing the story about Lindsey. There's no way Troy could have already seen her on TV and had the time to make the phone call.
Continuity mistake: Drew Barrymore gets hit with the baseball right below her left eye; yet, when she is on Jimmy Fallon's bed, it is her forehead that is swollen.