When Han Solo fires shots from his blaster at Darth Vader in Cloud City, the weapon zips into Vader's hand. To achieve this effect, director Irvin Kershner threw the gun across the table and the crew photographed a quick take of the gun flying through the air and then put a string on it, put it in the hands of Darth Vader and pulled it out with the camera upside down. See more...
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Star Wars: Episode V - The Empire Strikes Back (1980) - 36 revealing errors
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)
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).
When the stormtroopers are firing at Chewie and the droids and C3PO is yelling at R2D2 about the Millennium Falcon's hyper-drive, there is a shot of Chewie being shot at and the beam putting a hole in the wall. In one shot, it doesn't look much bigger than a pinpoint, but in the next shot, the team at Industrial Lights and Magic were kind enough to add a CGI burn mark, which contracts, almost disappears, and then warps as smoke passes in front of it, wrecking the illusion.