Revealing mistake: When Darth Vader and Ben are fighting, Darth's chestplate is on backwards and the belt "buckle" is as well. It appears that the entire shot was reversed. (01:26:20 - 01:27:15)
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 Captain Antilles and tosses his body to the wall. The "dead" Captain puts up both of his hands to avoid slamming his face into the wall. (00:05:40)
Revealing mistake: When the Tusken Raiders are seen raiding Luke's speeder, you can actually see the wheels or at least whatever is holding it up off the ground. (00:28:10)
Revealing mistake: On the Jawas' transporter, when R2D2 looks around in one of the shots, you can very clearly see the actor, Kenny Baker, inside through his eye hole.
Revealing mistake: When an Imperial Officer is running down the tunnel in the Detention Block towards Han, Luke and Chewbacca, Han clearly shoots the top-left hand corner of the tunnel and the Imperial Officer falls down the stairs as if he shot him. (01:14:55)
Suggested correction: This is definitely intentional, not an error. We see stormtroopers being killed by nearby shots throughout the movie, blasters having a smaller 'shock wave' is just a retcon.
Revealing mistake: In the Death Star corridor leading to the Millennium Falcon, just before Han says "Didn't we just leave this party?" you can see Carrie Fisher and Mark Hamill's shadows on the wall as they await their cue to run in. (01:31:30)
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: When Luke and C3PO are outside of the farm searching for R2D2, a droid passes in front of them. You can see lines in the sand, either to keep the robot in a straight line, or where the wheels of the robot have gone in earlier takes. (00:26:50)
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: When Darth Vader flies out of the Death Star you see the light falling on the back of the ship change as he clears the bay. Later, when he regains control of his spinning fighter the same shot with the falling shadow is used to show him flying away. (01:49:30)
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: When Luke first ignites his lightsaber, he turns it from the right side of the screen to the left side. When the blade is facing the screen, the lightsaber appears flatter than in the previous shot. (00:33:50)
Revealing mistake: When Greedo is threatening Han in the Cantina, there's a black object behind him. When Han shoots him, the object is smaller and some round things appear on the wall. It's visible in the screenshot of the dummy Greedo. (00:50:10)
Revealing mistake: When they are diving into the trash compactor on the Death Star, one can see the metal bars of the grid, that had previously been cut through, wobble like rubber when one of the team brushes against them. (01:18:20)
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: After Han Solo runs into the room full of stormtroopers, you can see rubber soles under Chewbacca's feet as he follows him. (01:24:15)
Revealing mistake: As Luke prepares the line and grappler to swing across the shaft, Leia provides cover fire. In one close-up, you can see a cartridge case being ejected from the 'blaster gun'.
Revealing mistake: Special Edition. When Han is speaking to Jabba in the hangar, when they are talking about the percentage, or something like that, Han says that 20% is too much. When he says that, his sleeve passes through Jabba's arm.
Revealing mistake: If you watch carefully before Han Solo shoots Greedo, you can see that Greedo has been replaced with a faker looking goofy dummy just before he explodes. (00:48:45)
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.