Audio problem: When Lando is punching desperately at the door's control panel in Cloud City, he says "The security code has been changed!" but his mouth doesn't move when he says it.
Continuity mistake: In the original 1980 release of The Empire Strikes Back, the group accompanied by Billy Dee Williams just walked from the deck with the Falcon inside the halls of the city. There is a man fixing a wall and next to that man is a window, the window shows a blue sky with a couple of buildings that were obviously drawn in the scenery. However in the last scene where Harrison Ford was hugging Billy Dee Williams the sky was orange. This shot only last for a second as the round small window is drawn wrong. But all of cloud city is a painting so its no surprise.
Revealing mistake: 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.
Continuity mistake: When the Millennium Falcon goes towards the bottom of Cloud City towards the end of the movie, we cut to the inside of the cockpit and Lando says "Look, someone's up there" when he notices Luke on the weather vane. At this point you can see Lando's right hand is on Chewbacca's seat when he says this, but when the camera is behind Lando in the next shot, his right hand is no longer on the seat.
Continuity mistake: At the rebel base, Leia is informed that there is nothing more can do about Han and Luke until the morning and agrees to the hangar doors being closed. In the wide shot, you can see the doors starting to close, although the code hasn't been put in yet. In the next shot the code is put in and the doors start closing again.
Visible crew/equipment: While in hot-pursuit, Han and Chewie are desperately trying to fix the Millennium Falcon (just after C-3PO informs them of the damaged hyperdrive, making light-speed impossible). In one shot Chewbacca has a bare human thumb, without gloves or makeup, and can be easily spotted in the lower left part of the screen. (00:38:00)
Continuity mistake: When Han, Leia and Chewbacca are captured by the Imperials in Cloud City, Lando says, "I'm sorry", to which Han replies, "I'm sorry too." At this point, Chewbacca is looking at Lando, but as the shot changes he is looking at Han, and then looks at Lando a second time..
Factual error: There are three major mistakes regarding gravity in the escape sequence in the asteroid field. Han and Chewie take the Millennium Falcon to refuge on an asteroid that, while visually huge, is still far too tiny to have sufficient gravity to allow humans to walk in anything like a normal fashion; yet, they walk normally both inside and outside of the ship. They also do not use pressurized suits outside of the ship, even though the asteroid's gravity should be far too weak to accumulate any significant atmospheric pressure; they use oxygen masks, but their blood should have boiled in near-zero atmospheric pressure. Finally, and most ridiculously, they fly straight down the giant cave worm's throat and land on the side of its throat (this is obvious in the shot where the Millennium Falcon lifts off and heads toward the toothy exit), and they get out and walk around on the side of its throat, which would mean the asteroid's gravity was impossibly perpendicular to its mass.