Continuity mistake: In the scene where R2-D2 is rolling down a path on Tatooine, under the eyes of scavenging Jawas, the path he is traveling on can be seen to end several feet in front of R2, and beyond it is a rough, rocky surface that R2-D2's wheels could not possibly traverse. In the next shot, R2 is rolling down a suddenly extended (or perhaps the same?) path towards capture.
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: 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.
Continuity mistake: When Han, Luke and Leia are trapped in the corridor in the detention block, a loose piece of hair on Leia's forehead keeps disappearing and reappearing between shots.
Continuity mistake: When you first see the Millennium Falcon, it is in the hangar. If you look close enough, you can see that only half of the ship is there. This is because it was too expensive to build a full-scale model.
Continuity mistake: When Han Solo and Luke Skywalker escort Chewbacca up to the Cell Bay to rescue Leia they are still wearing the stolen Stormtrooper uniforms, including white boots. Yet after they escape the trash compactor they are back in the boots they were wearing when they escaped Tatooine. While we can believe the rest of their outfit can fit under a Stormtrooper uniform, the boots wouldn't.
Continuity mistake: When Vader picks up the rebel in a choke grip, the legs of the stormtrooper in the doorway behind them are apart. In the shot of the rebel's feet, the trooper's legs are together. When Vader tosses the dead rebel into the wall, the trooper's legs are apart again.
Visible crew/equipment: After the heroes have escaped the trash compactor and we see them arguing outside of it. As they walk away and Han says "No reward is worth this," the shadow of the camera covers his whole face.
Continuity mistake: When Luke and his aunt and uncle are eating at the table, the white drink pitcher alternates from being next to Luke's cup to being across the table throughout the scene.
Continuity mistake: When Aunt Beru speaks to Luke about buying droids, there are many shrubs besides Luke. In the next shot, they are gone.
Deliberate mistake: When Obi Wan deactivates the Death Star's force field, there's an electronic gauge going down and a sign in clear English with the word POWER - as we've seen in shuttles and other places, they don't use English. The studio decided it was better to show "power" and "tractor beam" in English so that the audience would know what Obi-Wan was doing. The lettering we see in Jedi doesn't really affect the plot, so it doesn't need to be English. Still doesn't really fit though. This is fixed on the DVD.
Other mistake: In the beginning of the movie, when Leia is uploading a message into R2, we can see C3PO looking for R2. When they show C3PO, there are rebel soldiers behind him, running through the corridor, being shot at and falling down. If you look closely, you can see a black stripe vertically running down in the middle of the doorway.
Continuity mistake: When Vader tells Tarkin that Obi-Wan is in the Death Star, as Tarkin says, "The Jedi are extinct." he is standing between two chairs. In the next shot, he is standing behind one of the chairs. The distance between some of the chairs around the conference table changes during the scene as well.
Other mistake: After the escape from the Death Star, the first TIE apparently blows up, but behind the explosion we can see it still intact.
Continuity mistake: When Vader is strangling the Rebel aboard the blockade runner, the Rebel's hands keep changing position between shots, as he tries in vain to loosen Vader's grip.
Continuity mistake: In the control room when Obi Wan decides to leave them, the red beam on the radar behind him points down. When it cuts it is suddenly pointing up. It couldn't have moved that fast.
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.
Continuity mistake: When Luke is outside of the farm seaching for R2D2 with his binoculars, C3PO is not visible in them even when Luke turns in front of him.
Continuity mistake: When Luke is eating with his aunt and uncle, he takes a sip of blue milk, and then lowers his cup twice.
Visible crew/equipment: After the attack on the Death Star, when the pilots return to the base, the camera crew is visible on C-3PO's body.
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