When Harold is cut off by the orange bronco, the man inside screams "better luck tomorrow," a movie John Cho starred in 2002.
Kal Penn, who plays Kumar, is allergic to nuts. During the scene where he crashes through the ceiling at the station, a nut-based mixture was used for the dust that comes out of the heat vent. Upon entering the set that day, Penn had a severe allergic reaction, and had to be taken to a hospital. Later, a safer, nut-free concoction was used for the scene.
The director of 'Harold and Kumar' also directed 'Dude, Where's My Car?'. This is given a nod near the end of the film where the guys meet up with Neil Patrick Harris again, Harold says "Dude where's my car?"
Kal Penn, who plays Kumar, is a vegetarian, so special veggie Castles had to be made for him.