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. (00:38:00 - 00:41:00)
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.
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. (01:24:30)
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 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. (00:30:30)
Factual error: Ben talked about going down to Red Sox spring training during "Easter vacation." Boston schools do not get Easter vacation. They take a week off in February and a week off in April. February is too early for spring training (and Easter, for that matter), while April vacation is during the regular season. (00:27:40)