While Eunice decides to take Howard's rocks back to his room, she hurriedly puts her wig on and it teeters on her head messily. When she opens her door and walks down the hall, her wig is neat and fitting perfectly.
When Howard and Judy are arguing on parallel escalators, he is walking up a down escalator while she is calmly riding an up escalator stepping down slightly as she walks so that they remain side by side. A second later, the focus shifts to a point of view shot from Howard's angle, but it shows his escalator going up. The focus shifts again, and Howard calmly steps off the top of his escalator three or four steps ahead of Judy, despite the fact that she should have had the easier ride.
When they are traveling inside the Chinese dragon and the camera zooms in on their faces, both of Howard's hands are visible holding the horizontal bar inside the dragon's head, but in the wide angle views, both before and after, one of his hands is on the base of the vertical pole holding the dragon head up from the bicycle.
When the cars chase the three wheeled bike, they drive from a hill with a STOP-sign. On the right we can see some parked cars (Chevrolet Corvair, Ford, Pinto and a Rover). In the next shot, when the Cadillac limo passes, the three cars are disappeared and changed in a 1971 Chevrolet Vega coupe, Chevrolet Caprice coupe, Triumph GT-6.
When Mr. Jones is following Mr. Smith at the beginning of the movie. Mr. Jones gets into a cab where the cab driver is very noticeably bald. But when he gets out later on, the cab driver has a lot of hair blowing in the breeze.
Contrary to popular belief, VW beetles do not float on water. They may float for a few minutes due to their structure, but they soon sink as any other car would. The VW driven into the bay sank to the bottom, and a second had to be used for the scene where the car is floating.
Throughout the film Howard strikes several rocks with tuning forks, and then listens to the tuning fork as if he's expecting a different tone when he hits different rocks. Tuning forks are made to resonate at a fixed pitch, so no matter what object is struck with the fork, it will always sound the same.
Howard mentions that his theory is that ancient humans used rocks to create prehistoric music 7 million years ago, but that is the time when chimpanzees and primitive humans split on the evolutionary tree, so at that time primitive humans were little more than chimps. Hardly able to create, or even appreciate, any type of "music." Hugh (Kenneth Mars) says that Howard is studying "Neanderthals", which evolved roughly 250,000 years ago (and could probably create the music with rocks that Howard postulates).
Throughout the car chase (near the end of the movie), the "Just Married" VW is a post-1967 Beetle. However, it is a pre-1967 Beetle that is later seen floating in San Francisco Bay. This was due to the fact that the first car sank into the bay at the end of the chase scene, necessitating a second car "prop" for the "floating beetle" shot (Ref: Peter B in the DVD commentary).
As the bike begins to roll backward down the hill, the pedals force Judy's feet to turn backward, indicating it has a Derailleur-type hub, which can only freewheel in the forward direction. In the longer shot that follows, her feet on the pedals are not moving, which would require that the chain be removed.
When the VW Bus is struck repeatedly during the car chase sand can be seen spilling out of the engine compartment in the back. This was obviously a junk van with the engine and powertrain removed and the sand added presumably to make up for the missing weight. The bus, when rammed by the black 1969 Cadillac DeVille convertible, is not the same one seen earlier when rammed by a 1953 Cadillac Fleetwood 75 limousine and the taxicab. This particular bus was a prop.
The goof item below may give away important plot points.
When Howard finds Judy sleeping in the top floor of the hotel, she is wearing a purple top and black slacks. Afterwards, she is wearing her white pantsuit from the night before. However, at this point, her bag has already been switched with one of the others, so she couldn't have a change of clothes.