The Opening Day sequence was filmed on September 4, 2004 with Stephen King throwing out the first pitch; the Red Sox lost that game, ending a 10-game winning streak and King was blamed for it in the Boston Globe.
Jessamy Finet (Theresa) is an actual, die-hard Red Sox fan who appeared in Still We Believe: The Boston Red Sox Movie (2004), a documentary about the Red Sox's heartbreaking 2003 season. Jimmy Fallon saw the film and convinced producers to cast her for the role of Theresa.
While filming in Toronto, Ontario Canada, Drew Barrymore was asked to throw the first pitch of the game. It barely made it to the plate and she was extremely embarrassed.
All the events in the film except the Red Sox 8-7 victory over the Yankees are real.
Steve Sweeney was originally slated to be the scalper, but had pneumonia. Peter Farrelly then pulled Ken Rogerson, who was just an extra at the time, over and said, "Your part just got bigger."