Continuity mistake: When Luke and Obi-Wan enter Docking Bay 94 in Mos Eisley, as the first shot ends, C-3PO is turned to his left and is standing right behind Obi-Wan on Obi-Wan's left. In the next shot, Threepio is turned to his right, and is standing further away from Obi-Wan, behind him on Obi-Wan's right. (00:52:25)
Continuity mistake: Widescreen version: In the trash compactor, as Han says: "What an incredible smell you've discovered." Leia is leaning against some garbage, and then turns her head. In the next shot, she is no longer leaning against the garbage, and her head is turned another way. (01:16:05)
Continuity mistake: When Luke starts to lead C-3PO and R5-D4 to the garage, some of the droids in front of the sandcrawler repeatedly change position. For example, as Luke says, "And the red one, come on." R2-D2 is standing next to the boarding gantry, but in the previous and following shot, he was standing by the front treads.
Continuity mistake: In the original version of the film, when Luke and C-3PO are searching for R2-D2 outside the moisture farm, in the shot where Luke says, "It's too dangerous with all the Sand People around. We'll have to wait until morning", the sky changes colour. This has been fixed in the Special Edition.
Continuity mistake: When Han yells at Chewbacca to "Get in there, you big furry ape", Chewie steps in. It cuts away, then cuts back and Chewie steps in a second time.
Continuity mistake: When the Rebel fleet is on its way to the Death Star, there's a shot where the fighters are seen from behind as they approach the Death Star. Given its tremendous size, and the rate at which it is growing larger to our view, their rate of approach must be incredible. But when Red Leader calls, "Accelerate to attack speed", when you consider their speed with their surroundings, they are now going much slower.
Revealing mistake: When Gold Leader enters the Death Star trench, there's a shot from his cockpit. When Luke, Wedge and Biggs enter the trench later, the same shot is used again. This can be seen from a flash of light accompanied by the sound of a shot, followed by three green laserbolts, then another flash/shot and more laserbolts.
Continuity mistake: In the scene where Alderaan is destroyed, the "buns" on each side of Leia's head are positioned higher up than in the previous scene she was, so that her earlobes are visible. When Luke and Han rescue her, her hair is back to normal. She's held prisoner in a cell, so it's not likely that she could or would have changed her hairstyle.
Continuity mistake: When Luke first tries his father's lightsaber in Ben's house, it has a blue color but when he is practising on the Millennium falcon, it now has a white or sometimes light blue color. It has nothing to do with lighting because light sabers always have a bright color in the rest of the movie, even in the darkness.