In the scene where Gary is playing a game on his PlayStation 2 with Brooke's date, Brooke can be seen reading a magazine with Angelina Jolie's face on the cover. At the time, Jennifer Aniston had just divorced Brad Pitt who left her for Angelina Jolie.

The song that John Michael Higgins sings at the dinner table (with both families) is "Owner of a Lonely Heart" by the band Yes. Additionally, Higgins is credited as 'Vocal Arranger' for this film.

Jennifer Aniston wore Generra Jeans throughout the film.

In the opening scene at Wrigley Field, Jon Favreau makes a reference to going to "Wiener Circle" to have hot dogs after the game. Wiener Circle is a hot dog stand famous for the staff yelling and cursing at you when you order. If you do not return the favor, they do not take your order. Generally, though, this only happens late at night on the weekends.

Jennifer Aniston's character Brooke gets a "Telly Savalas" wax job during the movie. In real life, Savalas was Aniston's godfather.

Vince Vaughn's parents have bit roles in the movie. His mother plays a tourist and his father plays Jennifer Aniston's dad

Brad Ausmus, Todd Hollandsworth, Rickie Weeks, and Derrek Lee are a few of the actual Major League Baseball players seen throughout the movie at Wrigley Field or on Gary's television.

The Tone Rangers is the name of an actual a cappella group in Washington D.C.

The original ending didn't go down well with test audience, as Jennifer Aniston fans found it upsetting. As a result Universal Pictures had to re-shoot. According to the Chicago Tribune, the first ending made basically the same point in a slightly different way.

This is the fourth time Vince Vaughn and Jon Favreau have been in a movie together (after Swingers (1996), Made (2001), and Rudy (1993)).

The original ending featuring Gary and Brooke running into each other at an art fair each dating someone that looked identical to the other (Brooke's date looked like Gary, and Gary's like Brooke) was scrapped when it tested poorly. A new ending was filmed with the same result but featured Gary and Brooke running into each other on the street alone. In both versions the two remained broken up.

The number twelve was repeated many times in the movie.

In some scenes, Gary plays video games on his PlayStation 2. The list of games he plays includes Grand Theft Auto: San Andreas (2004), Fight Night Round 3 (2006) and likely the 2006 edition of Madden NFL 2006 (2005).

The Old97's are performing their song 'Time Bomb' while Brooke is waiting for Gary at the concert. The Old97's also played the same song during the opening credits of another Vince Vaughn movie, Clay Pigeons (1998).

Vince Vaughn gained 26 pounds for the role. Vaughn was reported to have gained weight as a result of his recent decision to quit smoking.