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 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: 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.
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)
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: 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)