Continuity mistake: Darth Vader fires at Luke's X-wing and blows R2D2's top off. Half a second later it shows no damage at all. It's not even scorched. When Luke lands, R2D2's head is destroyed and it's scorched all over. (01:56:15)
Visible crew/equipment: When Leia and the others come back to the rebel base, she says "there's no time for sorrows." The camera is moving during this shot and its shadow is cast on the actors.
Continuity mistake: While Luke climbs the ladder to get into the X-wing's cockpit he asks R2 if the little droid is okay, then as Luke steps up he's holding his gloves in his right hand, but it cuts to the next shot as Luke leaps into the cockpit and the gloves are now suddenly grasped in his left hand. (01:43:50)
Continuity mistake: When Red 10 is destroyed by a TIE fighter during the trench run, inside the cockpit as the craft starts to burn up and explode the pilot and the helmet are completely different to that of Red 10 when seen earlier.
Continuity mistake: At the dinner table, Luke picks up the container in his left hand to pour. A moment later, he picks it up in his right hand. Also, as Owen explains about Obi-Wan, you see Luke's left hand lifting then lowering the glass. When the shot cuts to Luke's face, his right hand is making the motions.
Continuity mistake: The oil streak on Threepio's left, then right, shoulder has totally disappeared when he's seen in the Sand Crawler.
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