(at around 43 mins) When Harris and Proctor enter The Blue Oyster, Proctor acts like he is unfamiliar with the venue. This is despite the fact that in Scuola di polizia 2: Prima missione (1985), Proctor recited the bar's street address from memory, while in Scuola di polizia 3: Tutto da rifare (1986), a naked Proctor stumbled into the same building while trying to hide.
When House sits on the bench, catapulting Sweetchuck into the basketball hoop, a wide shot shows that the bench is several feet away from the wall where the basketball hoop is, making it impossible to enter the hoop from underneath at that position.
After speaking with Mahoney, we see Harris inadvertently walk into the shower room. There is an immediate scream of surprise and Harris exits the room drenched. Later on in another scene with Harris, we see that the showers are in individual, unmistakable shower stalls and are a distance from the shower room door. Harris would have (1) recognized this as a shower room and (2) even if he hadn't, there's no shower head directly inside the shower room door to walk under right after entering the room as Harris evidently did.
In the television version of the movie, Todd (Michael McManus) is thrown from the hood of a police car, and is apprehended by Callahan. Although she applies handcuffs to Todd in close-up, a wide shot reveals the handcuffs are secured around only one of his wrists, as Todd is holding the unsecured cuff in his free hand behind his back.
Positioning of extras changes when Kyle skateboards through the mall crowd while being chased by Sgt. Copeland.
The official movie poster depicts Michael Winslow with a moustache, despite him not having one in the film. This same goof was made in Scuola di polizia 5: Destinazione Miami (1988).
The stadium they lift Proctor into is clearly being constructed on the outside, but he is lifted into a full stadium as a game is about to start.
When the 44-Magnum goes off while it's in Mrs. Feldman's hands, it doesn't kick upwards like it's supposed to.
Proctor's Port-o-Potty is lowered into the stadium full of people, and the housing of the unit is lifted, leaving the base and Proctor. When the music plays, Proctor stands up and you can briefly see his underwear (which he wouldn't be wearing because he was using the toilet).
When Proctor (Lance Kinsey) grabs picks up his hamburger from the table you can clearly see a tube under his sleeve. Then he bites the hamburger and squirts ketchup onto the lower back of another officer who's standing next to him.
Proctor bites into a hamburger and squirts ketchup onto the pants of a fellow officer a few feet away. One, the container it squirts out of can clearly be seen between the hamburger buns, and two, ketchup otherwise is not a projectile condiment.
At the start of the film, Jones and Mahoney's dancing is out of sync with the Citizens on Patrol theme song. Michael Winslow's sound effects are also dubbed over by the song.
When Copeland and his partner have Arnie and Kyle blocked off in the alley, Copeland's partner's voice is heard radioing Captain Harris, although his lips are not moving, nor does he even pick up the radio.
When Mrs. Kirkland pulls Bud by the hair, Bud's voice can be heard reacting, but his mouth does not move.
(At 7:59) When Zed yells, "Let's do some poetry," his mouth is not seen moving.
During the climactic chase scene at the air show, several lines of dialogue are heard for a number of different characters, although their mouths are not seen moving.
When Zed and Sweetchuck are on the biplane, right before bailing out, the pilot can be seen on the back seat.
During the skate scene, after Tommy Guerrero passes by the camera, a shadow of the cinematographer is seen before panning up to Arnie on the ledge.
When Commandant Lassard's golf cart is making a wheelie, you can easily see the support wheels needed to do that stunt.
In London, Commandant Lassard refers to "Chief Hurst", though Hurst has already been promoted to Commissioner.
If you look at Harris's epaulets, his Captain's bars are not properly attached. They should be at the end, not where they are on his uniform.