Visible crew/equipment: When Carol-Anne is running away from Kane's voice in the garage, she ends up standing in a puddle, only to have hands pull her in seconds later. But when the camera moves down Carol-Anne's body to the puddle, you can see the platform the actress is standing on.
Factual error: When Bruce and Patricia are in a lift after being chased by the cars in the garage, the lift rapidly descends but the two people in the lift are seen struggling on the floor when they should just about be on the ceiling due to the speed of the lift's descent.
Revealing mistake: When Carol Anne has been hypnotized by the shrink, a hand breaks through the desk and throws a coffee mug through the mirror. You can briefly see the reflection of the plexi-glass protecting the actors behind the mirror as the mirror breaks.
Continuity mistake: When Pat and Bruce are in the garage being chased by the cars covered in ice and snow, the cars pull out to face them and you can see that the windscreens on the cars have tiny spy holes for the drivers to see through. In the next shot, when the cars are still and parked, the spy holes have been covered up.
Continuity mistake: At the end of the movie, when Bruce is carrying Carol Anne out of the mirror, you can see that Carol Anne is wearing some type of heel sneakers and when they come out of the mirror, her pajamas have feet.
Visible crew/equipment: When Donna is in the bathroom, before she goes to the party, Carol Anne comes in to say that she looks great. When they walk out of the bathroom, you can see a crew member in the mirror.
Revealing mistake: When Scott (covered in ice) is ejected from the pool in front of Bruce, watch carefully. In the first few shots, he has some pretty convincing "ice" prosthetics attached to his body. When the camera cuts after he runs into the door, however, suddenly he's merely covered in really cheap-looking foam/slime that doesn't resemble ice in the slightest. And it's not supposed to be ectoplasm, it's supposed to be ice.