Continuity mistake: When Harold and Kumar arrive in Texas, the rear license plate on their car is a Texas plate. The car's front plate is from Virginia when they are in front of the whorehouse. The rear plate changes back to Virginia once they get to the house.
Continuity mistake: In the scene where Harold and Kumar are on the airplane to Amsterdam and Harold is watching a movie on his laptop while Kumar sleeps, Kumar has a pillow behind his head. During the scene the pillow changes (from all light blue colored to light blue with a dark blue stripe) and then later the pillow disappears when Kumar goes to get up.
Continuity mistake: In the beginning, Harold has a damaged face and a bandage on the top on his head, but when they arrive on the plane about 2 hours later, his face is not damaged at all and his bandage is gone.
Continuity mistake: The back passenger window gets blown off by the shotgun in the whorehouse. But in a later, different scene, the window is still there, just rolled down half way.
Visible crew/equipment: When Kumar peels out backwards at the whorehouse, you can see the tow line attached to the undercarriage go slack. It disappears when the camera angle changes. It's also a revealing mistake in that it shows how they were able to do the stunt with the actors in the car.