In the scene where Brennan and Dale are in their room showing each other their night vision goggles and autographed sword, the camera pans around while the characters are walking, and you can see a Hugalo's Pizza car light on top of the chest of drawers in the room. Will Ferrell worked for Hugalo's Pizza in "Talladega Nights" (which starred both Will Ferrell and John C. Reilly) after he lost his job as a race car driver.
Director Adam McKay states on the commentary that they originally went to the island of Catalina for location searches and discovered that he did not really like the island. They instead shot on the coast and during several shots, the island of Catalina can be seen. McKay chose not to have it digitally edited out as a joke.
The name of the high school which Brennan performs during the talent show and gets made fun of by Derek is "Great Valley High School" which is the name of the actual high school director Adam McKay attended.
Brennan says he smoked pot with Johnny Hopkins and Sloan Kettering. There is a Memorial Sloan-Kettering Cancer Center in New York which is also mentioned in the lyrics to the opening song "A-Punk" by the New York band Vampire Weekend. And although it's not exactly called "Johnny," the famous Johns Hopkins hospital is in Baltimore.