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.