Visible crew/equipment: When Johnny is spinning the two robbers around, you can tell that he is actually holding dummies. In the close up shots of the robbers, you can see them kicking their legs, but in the wide shots of the room, they are rigid and lifeless.
Continuity mistake: About 40 minutes from the end (I think - Michelle Pfeiffer is on a beach with Dean Stockwell), a boom mike comes in from the bottom of the screen very obviously, taking up about a third of the screen.
Continuity mistake: When Frank sees what he thinks is the Ghost of Christmas Future in the elevator, he flips out and accidentally clocks Grace in the jaw. She puts her right hand to her face, but in the next shot she's holding her left hand up.
Continuity mistake: The strings of lights for the reindeer reins change continuously from one strand to two.
Audio problem: In one scene where Jackson is caught by some bad guys you can see (and hear) that a couple of times when Jackson laughs it is badly dubbed over.
Character mistake: Watson says that there are fifteen windows on the front of 221B Baker Street, but if you look, there are only twelve.
Other mistake: As the brown truck drives away in the scene after the man working in the newspaper office is "harpooned" by a Critter, if you look closely as the truck turns a corner you can see some Critters rolling after it. Well, after they roll past the truck in the bottom left of the screen you can see the Critters roll right smack into another parked car. Obviously the crew forgot to add a turning feature to their Critter balls (which were just some fur taped to a basket ball).
Factual error: At the end, when Cyndi Lauper touches the glowing pyramid, goes into a trance, and starts translating the inscription, Jeff Goldblum pulls her off and she doesn't finish it. One of the bad guys says that what Cyndi already said will be enough to help him decode the rest and that it's just a matter of "cartography" now. He should have said "cryptography" (skill of decoding secret writings) and not "cartography" (which has to do with making maps).
Visible crew/equipment: As Zeebo is kissing Tanya, the device used to make his ears wiggle is briefly visible.
Continuity mistake: When Celeste explains to the Elders why Earth should be saved she says, "They have sneezes", but doesn't explain what they are. The Elders don't look like they recognize the term. Later, one of the Elders sneezes, and says, "We don't have sneezes here." Now how could he know that he just sneezed, if nobody explained it to him?
Visible crew/equipment: In the scene after the storm, the ship moves in the sand. Look at its front and you'll be able to see a wire that moves the ship. It's very visible.
Visible crew/equipment: In the bathroom, when Mike's ex-girlfriend is getting undressed to take a shower a long-necked clown jumps from out of the toilet. Anyone, and I mean anyone, can clearly see the arm of a white guy sticking out of the bottom of the clown's neck controlling the puppet.
Continuity mistake: Sergeant Parker is thrown through the air with his boots on, yet we are shown the dead hobgoblins beside his smoldering boots.
Continuity mistake: Jesse sneaks downstairs so that he can go out, but the doorbell rings, so he hides in a closet. As he walks down the stairs, there is a table with a white pot on the edge of it. In the next shot it's in the centre. (00:17:55)
Continuity mistake: When Jackie, Tim and Luke are at the bar, the bartender places a glass of milk beside Jackie's right hand. When it cuts, it's between his hands. (00:36:15)
Factual error: Ivan Danko's police uniform is a combination of at least six different uniforms. His hat is a Moscow Commander's Special and he also wears a Parade Band Musician axelbant.
Continuity mistake: In the first club session, where Winkelmann presides, a pair of glasses appears in the pocket of his jacket right before he puts them on. (00:10:55)
Audio problem: 99% of Mooch's drumming is not in synch with the drumming sounds, especially the ride and crash cymbals.
Continuity mistake: Right before Steve Martin and Michael Caine make their bet, Steve walks up to Michael sitting at a little white table. Every time the camera goes back to face Michael his handkerchief in the breast pocket of his jacket goes from sticking way out of his jacket, to barely visible.
Visible crew/equipment: Near the end when Wilford and Don and Kitty are about to sneak into the Oceanic Center, they are scoping out the scene. As they turn the corner, the shot changes to a shot mounted on the hood looking right at Wilford. You can make out the top of the camera's shadow near the right windshield wiper. (01:30:45)