Continuity mistake: When a confrontional alien thug threatens Luke and Ben interferes, the thug has his hand on Luke's upper arm. In the next shot, his hand is on Luke's shoulder. (00:45:00)
Continuity mistake: When Han is about to fire his blaster at the door of the garbage compactor, Chewie moves away from the door. In the next shot, he is back in his previous position, and moves away from the door again. (01:16:10)
Continuity mistake: When Han and Leia try to use a pole to stop the trash compactor, Leia says, "Help me." Two shots later, Han can be seen lifting the pole. In the next shot, Leia is suddenly lifting it with him, and Han is holding the pole differently. (01:18:40)
Continuity mistake: After Owen buys C-3PO from the Jawas, Luke says, "All Right. Come on." and R2-D2 begins to turn his head/dome. In the next shot, his head is still turning, but his dome is in a different position from the previous shot, evident from the droid's features. (00:17:45)
Continuity mistake: After the droids leave the control room of the Death Star, there's a close-up of Luke in the trash compactor, pushing against some garbage. In the next shot, he is pushing against the wall. (01:19:45)
Continuity mistake: When the Millennium Falcon escapes from the Death Star, each time we see Chewbacca, the sky behind him is dark grey with a few stars rather than black and and full of stars. Some shots later, the sky is normal.
Continuity mistake: When the garbage compactor is almost completely closed, Chewie holds the walls with his weapon. In the next shot, he is standing by the wall crying and in the other shot, he is holding the walls again.
Continuity mistake: After Darth Vader kills Obi-Wan Kenobi, Luke shouts "No!" and then there is a gunfight with stormtroopers. During this, R2-D2 is going up the ramp into the Millennium Falcon. But a few shots later, when Han Solo says "Come on!", you can see R2-D2 at the bottom of the ramp and he's going back into the Falcon for a second time.
Continuity mistake: In the trash compactor, Han says, "One thing's for sure, we're all going to be a lot thinner." In the next shot, Han and Leia are both standing in a totally different position. For example, both of them are looking in another direction, and Han's left hand is in a different position. (01:20:15)
Continuity mistake: In the scene where Luke finds the remains of his aunt and uncle, in the shots from behind Luke, the wind is blowing from his left. In the shots facing him, the wind is blowing from his right.
Continuity mistake: During the conference in the Death Star, the back of Tarkin's chair is higher in the wideshots than in the close-ups.
Continuity mistake: Luke is on the Millennium Falcon practicing with the floating orb. When he is done, he goes to shut off his lightsaber and as it turns off he jumps to the right due to a dodgy edit.
Continuity mistake: When Han, Luke and Leia run into the group of stormtroopers, Leia's hair is messy. When Luke and Leia reach the chasm, her hair is back to normal. (01:24:00)
Continuity mistake: When Leia is recording her holo-message to Obi-Wan Kenobi, she pushes a button on R2D2 to finish the message but each time we see the actual message during the film, she turns a huge round button.
Continuity mistake: When Owen is talking to C3PO, the droid has a wire coming from his head to his back. It's not there in the rest of the film.
Continuity mistake: When Tarkin meets Leia in the Death Star, he touches her chin at a certain moment. His arm is bent, but it's impossible because he is too far from her in the previous and the next shot.
Continuity mistake: If you watch Leia while they're in the garbage compactor, her belt, at various points, has fallen down around her thigh. It moves from her thigh to her waist (depending on the camera angle) during the whole scene.
Continuity mistake: In the beginning of the first scene of the duel between Vader and Obi-Wan, their lightsabres are held in different angels between several shots.
Continuity mistake: The first time that Luke plays Leia's message from R2-D2, he jumps back (which looks fine), but C3PO flinches and slips off the little 4" or 5" ledge between him and R2.
Continuity mistake: In the Death Star, when they are running into the Millennium Falcon, Leia tells Luke to come. She is behind a piston of the door but the shot later, she is in front of it.
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