Other mistake: When the stormtroopers break into the control room, the stormtrooper on the right of the screen hits his head on the door frame. On the DVD release they've added a thump when he hits it. (01:18:55)
Continuity mistake: When the Jawas are walking toward Luke to take the damaged droid, there's no more smoke coming from it but in the following shot, there's a lot of smoke. (00:19:18)
Continuity mistake: A few seconds before the Detention Block shootout begins, the 2 Death Star Troopers pull out DH-17s but after the fight begins and it cuts back to the Trooper on the left, he is holding an E-11.
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