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.
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.
Toward the end of the movie, Vince Vaughn uses his favorite line when he tells the passengers on his tour boat about getting hopped up and making some bad decisions. He uses the same line at the beginning of his role in The Wedding Crashers.
The French movie poster hanging above the TV in Jennifer Aniston and Vince Vaughn's condo, "Un Tueur dans la Foule" is for the 1976 movie, "Two Minute Warning, starring Charlton Heston which was also released by Universal Studios.
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).
John Michael Higgins plays a member of an acapella group called the Tone Rangers in this movie. Several years later, he goes on to play an acapella competition commentator in the movies Pitch Perfect and Pitch Perfect 2.
The trivia item below may give away important plot points.
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.