Revealing mistake: In the scene where Axel throws the art dealer's henchman onto the buffet table, if you watch carefully (or use slow-mo) you can see the stunt double pulling off that manoeuvre. There is also a stunt double being thrown over the table.
Revealing mistake: When Michael Douglas attempts to swing on a vine over a deep abyss, the rock that he runs into on the other side moves like a marshmallow, indicating that it's a fake rock. (00:39:10)
Revealing mistake: There's a scene where Dana's building is falling apart and stones and stuff are falling to the street below. One of the huge boulders bounces off a wooden police barricade in the bottom left hand corner of the screen, and then off a person. (01:29:14)
Revealing mistake: When the TVs explode, the glass is very fake and "cardboardy".
Revealing mistake: When the professor is trying to find the gremlin, he is in the room with the movie playing. The projector that is playing the movie is actually moving backwards. (00:44:35 - 00:47:35)
Revealing mistake: When Harkin Banks almost loses control and skis through the lodge, the jump ramp is visible as the woman opens the door. (01:29:45)
Revealing mistake: When the guy on the horse pulls out his sword, he has trouble getting it out of the sheath at first and it bends severely. It is clear that it is not a real sword.
Revealing mistake: In the scene with the motorcycle riding on its rear tire, skid marks can be seen from previous takes.
Revealing mistake: The opening chase has a police car chasing a white Countach. We hear two officers talking to each other. There are times they are talking to others on the radio, as well. But every shot that allows us to see inside the patrol car, only a driver is in it. (00:04:15)
Revealing mistake: When Supergirl flies through the window to fight the invisible monster, half of the screen shows her jumping in a school uniform and the other half exiting with the Supergirl outfit. Watch closely and you'll notice that her halves don't match, giving the sense that she is missing part of her body.
Revealing mistake: When Carolyn and the Geek are in the parking lot across from her church, Carolyn gets some help as she climbs over the side and out of the Rolls-Royce - as opposed to using the door. As she makes her way over the side, you can see that she steps onto a step-stool or similar above-ground block before stepping down once more with her next step onto the pavement. There is no such step on the car itself.
Revealing mistake: When the sheriff is going to whip Sarah, he tells her to take down her drawers. Bad editing, you see her hand reach back and make the motion as if she is pulling down her panties but it is visible they are still up.
Revealing mistake: In the opening scene where the alien car is pulled to the side of the road by the cop you can see two identical sets of tire tracks in the dust in the same place - a sign that this scene was filmed more than once in the same location and the filmmakers didn't wipe away the tracks. As the car pulls out you can see another set of identical tire tracks further up the road as well.
Revealing mistake: When Chocolate Mousse throws the anchor with the rope attached into the air, if you watch the ground when they run away, you can see that there's suddenly a lot more rope there, revealing that it was filmed in two takes and the anchor wasn't really thrown directly above their heads. (01:11:20)
Revealing mistake: When Roger is in the bathroom at the urinal, he can see Edwina's reflection in the mirror (as she is, supposedly, inhabiting his body). However, when Roger lets out a particularly big exhale, Edwina's bangs blow slightly upwards, showing that the "mirror" effect is achieved by simply having her stand behind a wall right in front of him. (00:29:45)
Revealing mistake: In the scene when Turbo is sweeping in front of the grocery store, he begins dancing with a broom and you can see the wires that are holding the broom up, making it seem like the broom is floating.
Revealing mistake: When the Hotel manager walks in the recently vacated room, we see the piano collapse. If you look closely, you can tell that the front two legs were just pulled back under the piano causing it to collapse.
Revealing mistake: The scene with one of the girls in the Pi house undressing on the TV was filmed with a handheld camera. Lewis tells Lamar to pan the scene right which it does. Cut to the scene of the woman undressing and the scene is shaking, like someone is holding the camera. These should be stationary shots. We even see the camera that is filming the shot and it's strapped down with a periscope.
Revealing mistake: When Melvin jumps out of the window, the person who falls out isn't actor Mark Torgl but a stunt double.
Revealing mistake: The seen where Teddy is standing in rain talking to Charlotte in her car you can see how the rain followed his movement. There is a thicker spout of water dripping and you can see the outline of it.