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: When the dianoga grabs hold of Luke in the trash compactor, his legs are together. In the next shot, they are much further apart. (01:17:05)
Audio problem: When Luke, Han, Chewie, Ben and the droids are in the control room, and Luke says "but they're going to execute her," watch closely: his mouth doesn't move for half the sentence.
Continuity mistake: In the Cantina, the burly alien threatening Luke pulls out his blaster twice, once in the background when Luke crashes into the table, and once as Ben's lightsaber is heard being ignited. (00:45:05)
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: When Luke and his uncle buy C3-P0 and a white-and-red R5-D4 robot from the Jawas, the R5 unit blows up after travelling a few feet. There's a quick shot of R2-D2 back at the Jawa's transport, and behind R2 you can see the same white-and-red R5-D4 unit being worked on by the Jawas. In the next shot it is back over with Luke. (00:19:15)
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: After C-3PO is separated from R2-D2 on Tatooine, when he's walking among the sand dunes, the sky alternates between being clear and cloudy. (00:10:15)
Continuity mistake: In the trash compactor, as Han says, "I've got a bad feeling about this", Leia's arm is held straight, but in the next shot, it is bent. (01:18:15)
Other mistake: Just before Darth Vader enters the Blockade Runner, there are some kind of tiny white-green lasers in the air. They are sometimes in line with the normal red ones. The stormtroopers and the security guards of Leia are both using red lasers, so there is no reason to see green ones.
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.
Revealing mistake: When Ben and Luke are transporting C3P0 after the attack of the Tusken Raiders, the arm of Anthony Daniels, the actor that plays the droid, is visible during the transition to the next shot showing Ben's house.
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: Just after the scene where R2-D2 is riding shotgun on Luke's X-Wing fighter and is hit by a shot from Darth Vader, there's a shot of C3-PO standing beside Leia in the rebel HQ, and the dent on C3-PO's head is on the right side (it's on the left side throughout the rest of the movie). (01:56:20)
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".
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