Revealing mistake: During the part when the day reaches October 31st, various scenes show children and happenings with their Halloween masks throughout various portions of the country getting ready for the big give-away. Included with this is the scene showing Dr. Challis' two kids again sitting in front of the television with their masks on, but they are wearing the same exact clothes they had on when he had stopped over at their house 8 days earlier. This scene was obviously cutting-room floor from that previous scene.
Revealing mistake: During the scene right after Dr. Challis breaks into Silver Shamrock and comes upon the "old lady" (robot) in the chair knitting, it is easy to tell if you look at the movements of the hands before Challis grabs her, it is indeed a real person (probably a stunt-person) doing the knitting, and the hands are real. Then, when Challis grabs her and she comes apart, the hands and other body parts on the floor are now all plastic with gears.
Revealing mistake: In Buddy Kupfer's death scene as "little" Buddy falls to the floor and the creatures exit his mask, his hand moves across the floor in one shot after several creatures have already exited the mask (even the rattlesnake wouldn't have the power to do that) and he is obviously dead.
Revealing mistake: Watch closely during the scene where Ellie Grimbridge arrives at the hospital room to id her dad Mr. Grimbridge, who the night before had been murdered. As she says, "Let's get it over with", there is a door behind her. On the door is a tag which has the room number. The shadow on the door is about 5 inches BELOW the room number tag. A split second later, the camera switches to a shot of Dr. Challis watching Ellie, and then back to Ellie and the door. The shadow is now EVEN to the room number tag, meaning the scene was probably shot a half hour later, yet the sequence of happenings was only a few seconds.
Revealing mistake: At the beginning of the movie we see one of Cochran's assassins enter Mr. Grimbridge's room at the hospital to gouge out his eyes and rip his skull backward. The assassin then wipes the blood from his gloves on the curtains. However, when the camera pans back to Mr. Grimbridge, he is just squinting his eyes closed really hard, and if you look close, his eyelids flinch a little bit.