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: 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.