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: 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.
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.