Other mistake: When Vader first arrives at the station, he and the Commander are walking past rows of troops. If you look carefully, you will see a Stormtrooper in the back row with his helmet tipped so far forward it's almost off.
Other mistake: Widescreen version: When Luke and Vader enter the Emperor's throne room, the stars seen through the window behind the Emperor move a little faster than the stars seen through the two other windows. (01:26:35)
Other mistake: When the ground battle on Endor begins, Leia runs to the bunker, and as the doors close, she fires a shot. In the original version of the movie, the blaster smokes, but no laser beam comes out of it and no sound is heard (do not confuse this with the sound of the doors closing). In the Special Edition, the sound has been added, but the laser is still missing. (01:38:05)
Other mistake: During the final confrontation with Vader on the Death Star, Luke bashes a catwalk railing with his light saber several times, but there are no burns, marks, or indentations on it at all. Considering the power of a light saber (Qui Gon melted a steel door in Phantom Menace), some damage should be visible. Only when he hacks off Vader's hand does the light saber slice through the rail.
Other mistake: Look at the shots of the windows in the Emperor's chamber on the Death Star. When they are seen from a distance, the window frames are out of focus, but the stars remain in clear focus.
Other mistake: Widescreen version: When Han calls after Luke and Leia when they leave on the speeder bike, the head of the stormtrooper that attacks him can be seen in the lower right hand corner, as the actor is waiting for his cue. After Han calls after Luke and Leia, the stormtrooper suddenly pops up and attacks him. (00:56:15)
Other mistake: After the catwalk that Luke is standing on collapses, Vader walks down the stairs to find him. In the first shot, there's a shadow of a lightsaber blade on the right side of the steps, but Vader's sabre is extinguished at this point. It can't be the shadow of Luke's sabre. Widescreen version only. (01:46:00)
Other mistake: On Endor, after Han Solo's group was captured, C-3PO called out to the Empire's soldiers. One of them said "Bring those two down here!" Both C-3PO and R2-D2 were up there, but from the Imperials were standing, they could only see C-3PO. R2 was hidden behind a tree. They wouldn't know that he was there until they actually got close to the two.