Revealing mistake: On the Death Star when C-3PO makes the announcement that he and R2 should join the others back on the Millennium Falcon, as R2 begins to move, the poor droid's right leg becomes rooted and begins to pull away from its socket.
Revealing mistake: After they free Princess Leia, other stormtroopers make the door explode to enter the detention room. You can see that the door is pre-cut.
Revealing mistake: After Luke orders a drink in the Cantina, there's a shot of two purple-skinned aliens. One of them has big, purple, alien hands, but the other one is wearing white gloves over human hands.
Revealing mistake: When Han shoots a stormtrooper and runs after the rest of them, the stormtrooper he shot moves his leg out of the way so Han won't trip.
Revealing mistake: When Luke and Leia are trying to escape the Death Star and they are stopped in front of the chasm, the stormtroopers shoots on the door, but you can see the pyrotechnic charges and the explosions are not where the laser touches the door.
Revealing mistake: When Darth Vader releases the admiral and he slumps on the desk, the force of the impact makes the whole set wobble.
Revealing mistake: During Kenobi's final lightsaber battle with Vader, Kenobi tips his lightsaber downwards, right before a scene change. The white "glow" that should be there isn't, showing the stick-like prop the actors used in filming. There is a small white light at the tip, too. This was not fixed in the Special Edition, but it was finally fixed on the 2004 DVD. (01:31:25)
Revealing mistake: When the Millennium Falcon is attracted to the Death Star, there is a frontal view of it. At the end of that shot, the sky under the ship becomes suddenly bright and you can see the stick used to move it.
Revealing mistake: When the droids head towards Tatooine in their escape pod, a rotating imperial ship is shown in the pod's window, getting smaller as the pod moves away. Regrettably, the star field gets smaller at the same rate as the ship, something that would only happen if you were moving away from a photograph, painting etc. (00:06:55)
Revealing mistake: At the beginning of the film, when the Blockade Runner is shot at, some of the lasers seem to pass through the star destroyer.
Revealing mistake: In the shot where Han Solo and Luke jump down the garbage chute, Smoke outlines the end of the corridor, revealing it is a board with the scenery on it. (01:19:00)
Revealing mistake: In the trash compactor, after C-3PO realises that he turned off the comlink, Leia grabs the bar used to stop the compactor and her hand sinks in and puts a dent in the bar. When she loosens her grip, it returns to normal. No wonder it was useless at bracing the walls...
Revealing mistake: When C3PO is carrying a dead Jawa to the funeral pyre, it looks like it doesn't have a head, legs and arms. It's like a costume stuffed with a pillow. (00:41:45)
Revealing mistake: When Wedge leaves the Death Star trench, the green screen is visible behind him for a few frames. (01:54:38)
Revealing mistake: In the Death Star, when Chewbacca and Han arrive in the hangar, the shadow of Luke and Leia waiting for their signal is visible.
Revealing mistake: Many times during the film, when someone shoots with a blaster, the shot does not come from the gun. A good example is in the detention block of the Death Star. Luke shoots and the laser comes from 10 centimeters over the gun. Special edition.
Revealing mistake: For this one, you will need a letterbox copy that is not the special edition. In the final award scene, there is a long shot of Luke, Han, and Chewie walking through rows and rows of Rebel Troopers. If you look quickly, the troopers on both ends of the screen are cardboard cutouts. They are cropped off in the Pan and Scan version, and digitised in the special edition, so it has to be the right copy to see this mistake.
Revealing mistake: When the stormtroopers stun Leia, if you watch carefully, you can see that she begins to fall before the beam touches her.
Revealing mistake: When Luke and Leia are trying to pass the pit in the Death Star, Leia shoots at a stormtrooper. The shot passes through him without hurting him.
Revealing mistake: Special Edition Only: In one of the new Mos Eisley scenes, the speeder goes down a road with two Stormtroopers on each side. The troopers are just mirror images of the ones on the other side of the street. Their movements are exactly the same.
Answer: Presumably because the monster has pulled him through the lair of whatever they are standing on into whatever space the rest of the body of the monster lives in. Obviously, the monster doesn't live in the part of the part of the compactor that does the compacting or it would be already compacted.
Myridon