Revealing mistake: When the bats invade Dr. Dunwoody's bedroom, you can see the strings on one of them.
Revealing mistake: There is a wide shot of the ship were there is some equipment on the ice and some people moving around. The ship itself and the equipment cast shadows. But none of the moving people cast any shadows at all. (00:14:25)
Revealing mistake: As Ana is driving away from her home, when we see her car driving from a shot high above, we see a car crashing in to a gas station making a huge explosion. However the trees a few feet away from the explosion don't move. The shockwave would have made them react at least a bit. (00:09:20)
Revealing mistake: At one point in the movie, dumptrucks are sent in to contain the Mighty Peking Man. You can tell that these, like most of the movie are toys.
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: In the last scene of the film where the Reverend is holding up the glass jar with the mini-blob in it, you can see the mechanism being used to move the blob (look between the Reverend's fingers). (01:30:30)
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: When Bob and Ben prepare to gas the ant nest, an ant appears below them. But its legs move too slowly, compared to the speed in which it appears; it rather looks like it is driving across the ground on (hidden) wheels.
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)