When Harold and Kumar were walking to Freakshow's house, 'John Cho' was unaware that there was a German Shepherd in the cage next to them. When the dog barks, Cho shouts and grabs Kal Penn. This reaction was genuine.
As a "thank you" for all of the free advertising the film gave them, White Castle arranged to have collectible "Harold and Kumar" cups at all of their locations during the film's release. It marks the first time an R-rated comedy is advertised on fast food containers.
On the DVD version if you go to the Menu screen you'll see Harold and Kumar driving Harold's car. If you let it sit long enough, they'll start to complain. "Why haven't they picked one of the menu choices yet? There are only, what, four choices?" Wait awhile longer: "They STILL haven't picked? What should we do? Do you want me to take my clothes off?".
The characters Rosenberg and Goldstein are named for the characters Rosencrantz and Guildenstern in the William Shakespeare play "Hamlet". The writers hope to create an alternate film with these characters that takes place in the same time period much like the way that the Tom Stoppard play "Rosencrantz & Guildenstern Are Dead" takes place in the same time period as "Hamlet".
The real restaurant "Hot Dog Heaven" is only located in two cities in Ohio, Amherst and Avon Lake. The Producers had lunch in the original restaurant in Amherst one day and loved it so much they asked the owner to use the logo in the movie.
Neil Patrick Harris plays a character named "Neil Patrick Harris" and is billed as such rather than being billed "as himself." According to an interview on NPR, this was done to make clear that he plays a parody of himself.
The "dust" in the vent was made out of walnut powder, even though the crew had received a memo saying that Kal Penn is extremely allergic to nuts. Production had to be stopped for the day that they shot the scenes with Kumar falling out of the vent in the police station while Kal Penn had to go to the hospital. The powder was then remade with chocolate powder for the next round of shooting. (from DVD commentary)
On a promotional talk show appearance by Christopher Meloni, he explained that when writers Jon Hurwitz and Hayden Schlossberg approached him with the script, they said of the part "You we're the first person we thought of." After reading the introduction of his character, which simply describes him as "the ugliest man on Earth," he got the joke.
Harold shares a jail cell with Tarik, who tells him not to get riled up by people because "in the end, the universe tends to unfold as it should". He doesn't give credit, but Tarik is quoting the 1927 poem "Desiderata" by Max Ehrmann.
The pipe that Goldstein and Rosenberg smoke marijuana out of is made of a shofar. A shofar is a musical horn made from a kosher animal's horn (usually a ram) that is used on the Jewish holidays of Rosh Hashanah and Yom Kippur.
Although they did not work together, Kal Penn and 'John Cho' appeared in House M.D. (2004) TV show. Cho was House's patient in the first season, and Penn played doctor Lawrence Kutner from the fourth to the fifth season of the series.
Some of the books on Kumar's bookshelf include: "To Kill A Rabbi" by Reuben Slonim, "Spy Story" by Len Deighton, "The Grand Chessboard" by Zbigniew Brzezinski, "The Knight, Death and the Devil", "McMahon!" by Jim McMahon and Bob Verdi, and "Catch". Kumar also has a Fourth Edition MCAT prep book, but that's just the hiding place for his weed, as well as his supply of roaches to roll with.
The trivia item below may give away important plot points.
When Harold and Kumar finally make it to White Castle, they stumble upon Neil Patrick Harris. Who, midway through the film stole their car. They both make a reference to "Dude Where's My Car" (2000) also a stoner film. "Dude where's my car?" "Where's your car dude?"