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 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 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: 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: When the bugs crawl out of E.G. Marshall's mouth, it is obviously a dummy.
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: When Coach Janna hits Russ Thorn with the fire poker, it's very apparent that it's made out of rubber. (01:07:20)
Revealing mistake: When Meg Tilly is being drawn toward the casket by the strange pink light the telekinetic dead man Karl Raymar emanates you can see strings on her feet pulling her toward it.
Revealing mistake: During the autopsy scene, it's obvious that the Panther is still breathing when it's supposed to be dead.
Revealing mistake: When the Swamp Thing walks past a bush, it's camouflage netting.
Revealing mistake: When Alison discovers Nancy and Tom are dead you see Tom's dead face change in each closeup. When she rushes out of the door in a panic Tom's eyes move as she opens the door and then moves again as the door closes. (01:03:40)
Revealing mistake: Throughout the movie, practically every victim ends up with a broken, twisted neck. The "special effect" used for those deaths is...the actors wear their jackets backwards. It is made even more evident by the plot twist at the end.