Continuity mistake: Leia and Han become annoyed with C3PO's chattering, and Leia turns him off. However, if you watch him in the background, you can see his body move as he breathes.
Revealing mistake: Before Lando warns the people of Cloud City that the Empire has taken control of the city, Leia shoots a stormtrooper. The burn mark can be seen on the troopers armour before the laser beam hits him. (01:42:40)
Audio problem: When Han punches Lando, his hand never makes any contact with Lando's face, and the sound effect is heard after his hand goes past it. (01:29:20)
Continuity mistake: When Vader attempts to turn Luke to the dark side, he says, "With our combined strength, we can end this destructive conflict and bring order to the galaxy." As the second shot begins, Luke is standing much closer to the structure he's holding onto, than in the previous shot. (01:46:20)