Other mistake: In the original version, when the Rebel fleet reaches the Death Star, Lando says, "Fighters coming in" and the Millennium Falcon flies towards the screen. If you play it in slow motion when the Falcon leaves the screen, you can see two white figures shaped like TIE fighters on the Falcon's underside. As the Falcon leaves the shot, they disappear. This has been fixed in the Special Edition.
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. Play it in slow motion for confirmation.
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: 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.
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 lightsabre 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.