Continuity mistake: When everyone is getting in fighters for the final battle, they all have white helmets with blue insignias. However, they aren't wearing them when they're flying.
Continuity mistake: When Darth Vader is interrogating the captain of the Blockade Runner, the stormtrooper on the left behind them has his blaster at his waist but in the shot showing their legs, he has it at his chest. It's at his waist again some shots later.
Continuity mistake: Widescreen version only: After escaping the trash compactor, Leia says, "From now on, you do what I tell you", and Luke's left hand is up by his head. In the next shot, his right hand is up by his head and his left hand is down by his side.
Continuity mistake: After the droids land in the desert C3-PO has a streak of oil running down his left shoulder. A couple of scenes later when he is walking after he splits up from R2-D2 there is a long shot of him and he still has the streak on his left shoulder. There is an immediate close up and the streak switches to the right shoulder.
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.
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 Obi-Wan's spirit instructs Luke to run, Luke is holding his blaster with two hands, and then drops it down by his side, holding it with only one hand. In the next shot, Luke is again holding the blaster with two hands, and repeats the movement.
Continuity mistake: After the escape from the Death Star, when C3PO is stuck in electric wires, there's a shot of R2D2 with the black and red computer panels of the control room in the Death Star behind him.
Continuity mistake: Every storm trooper has 2 pipes with slots besides their masks, but the one that shoots Leia with the stunt gun doesn't have those slots.
Continuity mistake: When the rebel leaders are studying the tactical illustration of the Death Star, the superlaser's dish is positioned on the battle station's equator. The Death Star's dish is actually offset from its equator, in the station's northern hemisphere. This was because the early CGI used to create the tactical readout of the plans took so long to make that by the time it was finished, the Death Star's design had been changed to offset the superlaser dish, and due to 1970s technology, there was no time left before the movie's release to change the plans, which were accurate to an earlier design of the station.
Continuity mistake: When Luke returns after dinner, he comes back and finds C-3PO hiding. When Luke finds out that R2-D2 is gone, he runs out of the room and outside. But you see the stairs he runs up to get outside. When you see Luke arrive and takes out the micro-binoculars C-3PO is right behind him. There is no way that C-3PO is that quick to keep up with a younger Luke. What did he do run?
Continuity mistake: When Leia's holo-message is being played in Luke's garage, and C-3PO explains about R2-D2's restraining bolt, the background behind the hologram has been flipped.
Continuity mistake: The microphone on Red Leader's helmet switches from one side of his face to the other, then back again as he pulls out of the Death Star trench (just before he crashes). (Actually the entire shot is mirrored. Check out the scuff marks on the centre of his helmet, they change direction as well.) Other shots and pilots' mics switch too: Red Leader when he says, "All wings, report in", Wedge after a shot of Leia and C-3PO monitoring the battle, Luke when Red Leader says, "There's a lot of fire coming from the right side of that deflection tower" and when Luke says, "I'm hit, but not bad".
Continuity mistake: In the final battle, the leader of the fighters (not the group that was to blow up the Death Star, but the leader of Luke's group), got killed. Then it goes back to the home base where Leia, C3PO, etc are watching the action and you hear a transmission from that leader.
Continuity mistake: When they are searching for the monster in the garbage compactor, the garbage is shaking, except in one shot.
Continuity mistake: When the garbage compactor is going to crush them, Chewbacca takes his weapon to retain the walls. He takes it again in the next shot. [Duplicated mistake]
Continuity mistake: During the medal ceremony, there is a close-up of Han followed by a shot of Leia with a big smile. She turns to the right to get a medal, then there is a wider shot of her and she's not smiling.
Continuity mistake: When the walls of the trash compactor start moving, Han is holding the pole, and leans it against the wall to his left (we hear it hit the wall). In the next shot, the pole leaning on the wall to his right instead., and the right side of the pole moves up and down between the next few shots.
Continuity mistake: When Darth Vader kills Obi-Wan Kenobi, there is a gunfight between Luke, Han and stormtroopers. While this is happening, Vader is seen leaving the hallway, but in one shot he is back in the hallway standing on Obi-Wan's cloak.
Answer: While planning Star Wars, Lucas had a vague notion of doing a long series of movies inspired by old serials, then dropped that idea in favor of just one. When Star Wars became a phenomenon and sequels became feasible, Lucas revisited the idea. He thought of three trilogies along with some stand-alone "in-between" stories for a total of 12 films. By the time of The Empire Strikes Back's release, this was pared down to the 9 mainline films, going by interviews with Lucas and the cast at the time. By Return of the Jedi, Lucas had decided to end the saga there, with the option that he could revisit the first three at some later point. It's unclear if Lucas ever had any specific story ideas for the proposed sequel trilogy, and they never had any scripts. Producer Gary Kurtz suggested in an interview they would've been about Luke's twin sister (not Leia), though many fans are skeptical about just how much he would know about them. Of course since this question was asked a sequel trilogy was written and released.
TonyPH