In Cloud city, when Chewy finds C-3PO in pieces in what looks like a droid scrap room, you can see what looks like the upper half of IG-88 leaning up against the wall. It could in fact be IG-88, or just a droid from that series. See more...
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Star Wars: Episode V - The Empire Strikes Back (1980) - 183 mistakes
Directed by Irvin Kershner, starring Alec Guinness, Anthony Daniels, Billy Dee Williams, Carrie Fisher, David Prowse, Frank Oz, Harrison Ford, James Earl Jones, Kenny Baker, Mark Hamill, Peter Mayhew (add more)
Visible crew/equipment: This can only be seen in the widescreen version. When Han, Leia and Chewie venture out of the Falcon to investigate the cave they're hiding in on the asteroid, as they walk towards the left of the screen you can see the edge of the tarpaulin sheet they're standing on as well as the faint outline of a crew member trying to get out of the camera shot.
Revealing: When Leia and Lando are holding off the stormtroopers while Chewbacca and the droids escape into the Millennium Falcon, a stormtrooper comes running through the doorway. You can see the burn mark on his armour before Lando shoots him. When the trooper is shot, another trooper comes running through the door, also with a burn mark on his armour. Then Leia shoots another (or perhaps the same) trooper, and the burn mark is once again there before the trooper is hit (watch in slow motion).
Continuity: When the AT-AT that Luke destroyed with a thermal detonator falls over, Luke is nowhere to be seen, even though he should be visible, judging from his position in the previous shot. (As noted in another mistake, the AT-AT is suddenly further away from him after he falls to the ground, but he is still to close to be out of shot when it falls over.).