Factual error: When Harry is standing on the ceiling fan, the young boy pulls the lever backwards to increase the speed. If you read the side of the lever, you can see that the boy is actually pulling the lever to the slow direction. (00:40:30)
Continuity mistake: In the scene where Sean Connery is in prison & Matthew Broderick is visiting - the camera alternates between looking into the cell (at Sean) and looking out (at Matthew). Watch the hands that are holding on to the cell bars - they move each time you get a new shot.
Continuity mistake: The snowman hits his nose with a hammer, completely bending it in half. In the next shot it is suddenly magically straight again.
Factual error: In the opening scene when the guys are shooting craps, Little Quick shoots the large man squarely between the eyes. The man wasn't looking downward and it would have been impossible for a 4 foot tall kid to shoot a 6 foot tall man at that angle squarely in the forehead.
Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.
Suggested correction: It's entirely possible that he was able to borrow a pair of pants while down there.
I just watched the scene where Maurice and Brian pull the pranks to get the kids into trouble and he is in his pyjamas. Later in the monster world he is in a full outfit including a shirt, pants, and shoes. Then he goes back home and he is in his pyjamas again.
Continuity mistake: In the opening credits, when Smart gets inside the phone booth, the doors are not fully closed, but the immediate close-up angle shows them fully closed.
Other mistake: Near the end of the movie there is a montage scene of acceptance letters being printed followed by the reactions of the various people who received the letters. Despite these form letters being sent on the same day the date on some of the letters is April 11, 1988 and the date on others say April 11, 1989.
Continuity mistake: Barbara Carrera's Priscilla has shoulder long hair most of the time yet at one point she's sporting a ponytail that reaches her waist.
Continuity mistake: When Alex is speaking with Christopher in their dorm room she's wearing a pink top with the right shoulder down. Then it's pulled up in one shot before being back off the shoulder again.
Continuity mistake: When Lucas visits Meg at the juvenile home, he carries a stuffed toy monkey by the leg. When the camera angle changes, he is holding the monkey by the neck. (01:08:15)
Continuity mistake: Sammo Hung just left the bus to ride the bike with his cousin Jenny. They ride talking about Stallone and the USA, and the boxes of Nike shoes are tied to his bike differently when they arrive compared to when they started. (00:23:00)
Continuity mistake: John Ritter's beard goes from being real to fake throughout the movie.
Other mistake: At the beginning of the film, there's no way Angela would possibly have been able to know the exact moment the girl would have been walking down the street and what she'd be wearing so she could be wearing the exact outfit.
Continuity mistake: In the beach scene the boys are eating ice cream cones. The ice cream keeps changing size.
Continuity mistake: When Harry throws back his arm to break the bodybuilder's front door with the crowbar, the front door and the object mounted on the wall are in the sun but as the camera changes angles where he actually finishes his swing and breaks the glass on the door, suddenly the door and object on wall are fully in the shade.
Visible crew/equipment: When Mookie throws the trash can through Sal's window, the reverse shot from inside the restaurant reveals the main camera, in the lower left portion of the screen, moving in towards the window as it's smashed.