Continuity mistake: After Darth Vader kills Ben, there's a shot from the docking bay towards him. Vader's lightsaber is missing its red color. [This has been fixed in the 2004 DVD, but is still valid for VHS prints.] (01:28:20)
Continuity mistake: In the scene where Luke and Leia are about to swing across the canyon inside the Death Star, Leia shoots one last time. She hits the bulkhead behind the stormtrooper on the right and he falls forward. The trooper on the left continues firing (as seen in the screencap). (01:25:25)
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: When Obi-wan and Luke are watching the hologram of Princess Leia projected onto a small table, the objects on the table change position and number in each shot. (00:33:35)
Continuity mistake: After the Stormtrooper tells Darth Vader the Death Star plans are not in the main computer, when Vader lifts the Rebel off his feet by his throat as the Rebel tells him they intercepted no transmissions, take note of the leg plates on the Stormtrooper to Vader's right. In the wide shot, the Stormtrooper's left leg armor does not have the knee protector plate (it looks as if it's at his right knee because the extra switched them), but in the next closeup the knee plate appears at his left knee, as it should be. The trooper also appears to go from right handed to left handed weapon carry. (00:05:40)
Continuity mistake: After Luke gets pulled underwater in the trash compactor, he is shown with a piece of garbage on his forehead in one shot. In the next shot his forehead is clean, but in the following shot, the piece of garbage is back again. (01:21:35)
Continuity mistake: The first interior shot of the Millennium Falcon's cockpit shows small hanging dice. In every other shot, however, the dice are gone. (00:54:55)
Continuity mistake: Twice during the lightsaber duel, Ben and Vader inexplicably change sabers as the color jumps from one sword to another. Vader is swinging the blue sword while Ben holds the red one. (01:30:55)
Continuity mistake: When Leia shoots a hole in the detention area wall for their escape, the hole is not big enough to jump through, but when they all jump in, the hole is much bigger. (01:15:10)
Continuity mistake: After Darth Vader has killed Obi-Wan and walks towards the group of storm troopers onto the Millennium Falcon, Luke shoots the dock door control, successfully closing it, fully blocking Vader's path. In the shot immediately following the door is closing once again. (01:32:45)
Continuity mistake: In the scene after Carrie Fisher inputs the Death Star plans into R2-D2 the droid moves towards C3-PO along a corridor. As you watch you can follow R2's movements until the camera cuts away. If the shot were to continue it seems clear that R2 would have banged into a piece of set protruding from the side of the corridor. (00:05:15)
Continuity mistake: Sometimes, the red stripes on the wings on the underside and topside of Luke's X-wing change. For example, compare the shot where he brushes his hand along the underside of the wing of the ship prior to the battle, and the shot before he says, "I'm hit, but not bad."
Continuity mistake: In Mos Eisley, when Ben and Luke are interrogated by the stormtroopers, the background changes. At the beginning and at the end of that scene, the street is large and full of people and beasts, but at other times, the street is small.
Continuity mistake: Special Edition DVD. When Luke, Obi-Wan and droids arrive at space bar in Mos Eisley, notice the herder with white jacket followed by a dinosaur-creature on a leash and then the R2 unit. In the next shot the dinosaur and R2 are there, but the herder has disappeared. (00:42:20)
Continuity mistake: When Luke is in Obi-Wan's house trying Anakin's lightsaber, the coat behind him keeps appearing and disappearing. (00:32:15)
Continuity mistake: When a stormtrooper stands guard by the door of the control room, the door is low enough for someone to hit their head on. But when C-3PO wants to take R2-D2 "down to maintenance", there is plenty of clearance between the trooper's head and the bottom of the door. This has nothing to do with camera angles, as Threepio is the same height as the trooper, and walks out of the room with no problem. (01:22:40)
Continuity mistake: Towards the end of the film, the Death Star rounds the planet towards the moon in order to destroy it. The movie continually reverts back to the Death Star where a commander informs us of the time until the Death Star is in range of the rebel base. It will then show the panel with the Death Star's co-ordinates and how long until contact. However, it does not count down in minutes, it counts down in seconds. E.g the commander will say "40 minutes till contact" yet the screen will start at 40 seconds and count down, 39, 38, 37. Regardless of whether or not they use the same time system as us, the time would have run out far too quickly. (01:47:30)
Continuity mistake: During the battle between Obi-Wan and Darth Vader, there's a scorched slash mark on the wall to Obi-Wan's right before either saber makes contact with it. A few seconds later Darth slashes his saber in exactly that spot (we can see the scorch mark again before he hits it). (01:26:50)
Continuity mistake: When Darth Vader is talking to Princess Leia, the chain holding his cape keeps changing between over and under his mask.
Continuity mistake: When R2D2 and C3PO land on Tatooine, the escape pod is on the flank of a dune. When the stormtroopers are doing their investigation, the pod is suddenly on more or less flat sand.
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.