Revealing mistake: In the scene at the end of the very first battle, where Imperial troops have just taken control of Princess Leia's rebel ship, Darth Vader breaks the neck of the rebel leader and tosses his body to the wall. The "dead" rebel puts up both of his hands to avoid slamming his face into the wall. (00:05:40)
Revealing mistake: Sand people are up on a hill overlooking a valley below where Luke is passing by in his speeder. One of the sand people takes aim at Luke, but his companion stops him. Right before the end of the shot, as the speeder is traveling towards the lower right corner of the screen, it becomes transparent. This is a multiple exposure shot, and the path of the speeder wasn't quite lined-up right. (00:27:55)
Revealing mistake: In the scene where the "torture" robot goes to get information from the princess, if you look carefully on the side of the syringe you will read "monoject", which is an American hypodermic syringe manufacturer. (00:41:30)
Revealing mistake: You can see the arm of the crane used to lower the torture device through the door when it is going into Leia's room. (00:41:15)
Revealing mistake: During the final battle at the Death Star, nearly every time Darth Vader is shown in his ship's cockpit, David Prowse's eyes are visible through his helmet's lenses, because the combination of the red lenses in the helmet and the red lighting causes a transparent effect. (01:54:40)
Revealing mistake: At the start of the Vader vs. Ben lightsaber battle, when Ben turns his on, a cord can be seen that runs up Ben's sleeve.