Revealing mistake: Just before Charlie is stabbed you can see the "blood pack" under his shirt.
Revealing mistake: It is obvious a dummy was used when Steve is thrown from his easy chair into the kitchen cabinet.
Revealing mistake: When Chelsea is on the roof of the house and Kathy joins her, Kathy's hair is blowing in the wind, while Chelsea's stays perfectly still, the water in the lake is also perfectly still. Obviously the director had the actors do the scenes separately until we see them both together in which case both characters hair is blowing around. (01:27:40 - 02:00:00)
Revealing mistake: When the nurse from Keto throws himself out of the window - you see the arm of the dummy fly off as it hits the floor. When it cuts to him lying on the floor - both his arms are intact. (00:15:12)
Revealing mistake: In the scene where May showcases her doll, Soozy, to the blind children at the day care and they subsequently knock the doll case out of May's hands (obviously shattering the glass everywhere), take a close look at the first little girl to fall into the glass on the floor. You'll notice that there is blood all over her knees before she even falls on the ground. (00:56:55)
Revealing mistake: Dan goes through the fraternity's records, and sees Devon's face in photographs going back almost 100 years. Notice how badly Devon's face is superimposed onto the earliest photographs (it's hard not to).
Revealing mistake: Near the end when a naked John Jasper kills M, he staggers back and you can see a modesty pouch that is covering his private parts. (01:31:47)
Revealing mistake: At the beginning, the dead policeman can be seen rolling himself with his arms. (00:06:45)
Revealing mistake: On night 21 the time-stamp changes from 11:59:56 PM to 11:59:57 AM despite it not having turned midnight yet.
Revealing mistake: When Christina Ricci picks up the silver knife off the floor in her house and her hand starts to smoke, pay attention to the shadow. You can see smoke coming from her wrist before her hand touches the knife.
Revealing mistake: At the end of the movie, when the last "Graboid" appears, in one or two shots of him retracting back into the ground, you can tell the shots are reversed, as dirt is being sucked down in an unnatural way. It flies up and sort of "implodes" towards the creature. (01:23:50)
Revealing mistake: When the eye is flying toward Bobby Jo, you can see the rig holding the eye as it flies and the wire when it enters her mouth. (00:41:45)
Revealing mistake: When the bleached dead girl is lying on the table, you can see her breathing. (00:13:45)
Revealing mistake: As Ripley and Dillon lure the Alien into the piston chamber, Morse closes them just in time. Watch closely - he misses the switch. (01:38:15)
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 Robert Downey Jr. is taking Halle Berry's blood pressure, the needle doesn't move as he pumps the hand inflator.
Revealing mistake: When the father moves to close his dead daughter's eyes, the daughter's eyes close before the fathers fingers close them.
Revealing mistake: When the bugs crawl out of E.G. Marshall's mouth, it is obviously a dummy.
Revealing mistake: In the chase scene about an hour into the movie, Robert Loggia is shown entering the Liberty Tunnel, driving on a wet road. Note the vaulted round ceiling of the tunnel. Next, we see a series of shots of him in the tunnel, and then a shot of the sun rising over Pittsburgh as he is about to leave the tunnel. Here is the problem - the sunrise shot is taken from the Fort Pitt tunnel on a different day. Note the dry road, the barriers to the driver's left at the tunnel exit and the Fort Pitt bridge straight ahead. There is no place for the driver to make a left turn out of this tunnel. Also note that the POV is from the left lane. All the Liberty Tunnel shots show Loggia in the right lane. The next shot shows Loggia exiting the Liberty Tunnel and making a left turn from the right lane, causing a pile-up. (01:11:10 - 01:12:20)
Revealing mistake: The roller coaster used for the Devil's Flight is really the Corkscrew at Playland in Vancouver, where all three movies were filmed. If you look during the opening credits, you can see the name "Corkscrew" up in the lights above the coaster. After this, it says Devil's Flight during the rest of the roller coaster scenes.