Continuity mistake: Special Edition: When Han is talking to Jabba the Hutt, when Jabba begins to says, "What if everyone who smuggled for me dropped their cargo at the first sign of an Imperial starship?", Chewbacca can be seen walking behind them. In the next shot, he is suddenly walking next to a wall that was nowhere near him in the previous shot. In the following shot, he is back to his original position. (00:51:00)
Revealing mistake: When Wedge leaves the Death Star trench, the green screen is visible behind him for a few frames. (01:54:38)
Continuity mistake: During the conference in the Death Star, the back of Tarkin's chair is higher in the wideshots than in the close-ups.
Revealing mistake: Special Edition Only: In one of the new Mos Eisley scenes, the speeder goes down a road with two Stormtroopers on each side. The troopers are just mirror images of the ones on the other side of the street. Their movements are exactly the same.
Continuity mistake: In the beginning of the first scene of the duel between Vader and Obi-Wan, their lightsabres are held in different angels between several shots.
Continuity mistake: When the walls of the trash compactor start moving, the bar Leia uses to brace them is easily obtainable, but in the previous shots, it was partly buried in garbage.
Continuity mistake: Widescreen version only: When Luke activates his new lightsaber for the first time, the position of C-3PO's head is different from the previous shot. After he turns it off, Threepio's head is back to its original position.
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: Watch Luke's shadow when he walks toward his aunt to talk to her, and when he turns around. In the following shot, where he is walking toward the Jawa sandcrawler, his shadow is cast in another direction.
Continuity mistake: In the scene where Vader chokes the imperial officer with the force, watch him before he gets choked. On his left (your right) of his blue and red button things, he has a pocket with what looks like two pens sticking out. But in a later shot, only one "pen" is sticking out. Later, it goes back to two again.
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 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.
Revealing mistake: On the Death Star, before Luke and Leia try to cross the pit, Luke shoots a panel to close the door. Firstly, he shoots beside it, and secondly, after it explodes, no electronic components are visible in it.
Revealing mistake: During Darth Vader and Ben's lightsaber duel, sometimes you can see a white wire attached to Ben's lightsaber, for example after he says, "If you strike me down, I shall become more powerful than you can possibly imagine."
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: 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 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 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: 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)
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