Revealing mistake: When the chair moves across the kitchen floor, Craig T. Nelson goes to investigate it (after it has stopped moving). He goes up to the chair and flips it over. If you look closely, Craig almost hits the camera with the chair's legs. You can tell this has happened because the camera moves up very quickly and Craig looks straight at the camera crew with very big guilty eyes. This is more easily visible on the widescreen version of the film. (00:33:30)
Revealing mistake: When they're dancing on the bowling alleys, they couldn't be waving their bowling balls about as if they were made from foam - bowling balls are heavy!
Revealing mistake: When Chris is trying to escape out of one of the first floor windows, the shots you see from outside are of a stunt double with really bad frizzy hair. (01:14:40)
Revealing mistake: In the scene where Chuck finds Bill in the morgue vault listening to the recording of Chuck telling him to shut up, take note of Bill's thumb on the rewind button of the tape recorder. You can hear the tape recorder rewinding to replay the words, "Chuck to remind Bill to SHUT UP" multiple times but Bill's thumb does not move at all until he turns the recorder off. (00:30:00)
Revealing mistake: When the oldest son is being pursued by the unseen entity whilst the family is seeing the priest. You can see a crew member in green pants and brown shoes to the left as the camera perspective turns upside down.
Revealing mistake: When the Enterprise is leaving space dock, look at the right (starboard) side of the ship. You can see the shape of the mounting arm the model was mounted on, even though it was blacked out to look like space, and it also blocked out the bottom of the dock. The footage was reused from Star Trek: The Motion Picture.
Revealing mistake: When MacReady burns the one "Thing" with the split head, in one shot it walks by MacReady in the foreground, and you can tell in that shot that it is just a still dummy burning, as it isn't moving at all. It just sort of floats past with its arms at its sides, while in the previous and next shots it is wildly moving around, arms flailing.
Revealing mistake: During the Eddie and Nick fight scene, you can clearly see the stuntman for Eddie. He looks nothing like him.
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: Jen Moo cuts off the God Skills monk's ear, but as Ching picks it up, it strangely shows no opening for the ear canal.