Revealing mistake: When Jamie Lee Curtis falls into the fuel tanks, how could she be breathing? The gases would be too toxic for her to breath, and wouldn't she be blinded by the fuel?
Revealing mistake: When the guys are heading to the vortex in the cart, you can see the tracks from the previous takes imprinted in the grass.
Revealing mistake: When David is sitting in L.E. at the end and the camera is zooming in on his face his mask changes. In the very last shot of him just about to pull it off it's much wider/looser so it'll come off easier.
Revealing mistake: When they are climbing up from the subway tunnel into Madison Square Garden, there are lots of half-dead fish scattered around from when Godzilla was feeding. Right before they enter the arena, in the overhead shot looking down on the actors, there is a wiggling fish in the lower left portion of the screen. If you look closely, you can see a string attached to the fish. A crew member is obviously tugging on it from below, to make the fish move.
Revealing mistake: In the parking garage shoot out Arnold shoots the leg off a bad guy. While the bad guy is writhing in pain on the ground you can see his leg tucked back. (00:59:50)
Revealing mistake: When the raptor in the InGen compound calls for help, the shots of the raptors responding are shots taken from other scenes in the movie.
Revealing mistake: Apparently filmed inside an old warehouse, heavy masonry walls can be observed during some scenes inside what is supposed to be a spaceship.
Revealing mistake: When Francis emerges from the cave, he stands in the foreground, with the distant mountains and sun in the background. The sun creates a lens flare, which is an internal reflection within the camera lens. Yet Francis is in front of the flare. This shows that the actor was composited with the background via a traveling matte process.
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: 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: When the main guy is fighting the main girl in his room, the scene changes angles and you can CLEARLY see that her stunt double is a man!
Revealing mistake: After the second movie sign, when everyone gets into the theater and sits down, there's a very obvious cut, as all the characters cut immediately from the position that they sat down in to a completely different position. (00:28:05)
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 bleached dead girl is lying on the table, you can see her breathing. (00:13:45)
Revealing mistake: When the boys are racing away on their bicycles from the Feds, the stunt doubles for Elliott and his friends are obviously way too tall to be children.
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: 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 Ray, Winston and Egon are underground looking for the river of ooze, there is a scene where they break down a small wall of rocks. Ray then sits on one of the rocks and it bends under him - it's rubber or foam. (01:01:50)
Revealing mistake: The Eternia out-door scenes depicting the conquest of the Castle Grayskull area are actually one long shot subdivided into smaller parts. Look at the captured Eternian soldiers marched to the castle; they cover very little distance between the shots.
Revealing mistake: When the bugs crawl out of E.G. Marshall's mouth, it is obviously a dummy.