Revealing mistake: When Luke, Han, and Chewbacca attack the prison of the Death Star, in one shot, the end of the corridor is in a slope while it is supposed to be horizontal. (It looks to me like it's actually a flat panel painted to look like the corridor. The angle looks perfect for the opening shot of them entering the prison floor, it just looks "off" for any other angle.) This is fixed on DVD.
Audio problem: [Special Edition only] In the scene where Luke and Leia swings across the missing bridge, a number of the shots fired by Leia are "bangs" instead of the laser "zap" sound. The bang is probably from the prop gun used (it shot blanks) and was never dubbed over (until the 2004 DVD release).
Revealing mistake: When Luke is first practicing with his lightsaber on the Millennium Falcon against the remote, the zapping blasts that it sends at him are in 99% of the case headed nowhere near his body, but disappear in line with it [very poor FX work] and he flinches.
Continuity mistake: In the cantina, the alien who eventually gets his arm cut off is hassling Luke. In one shot he puts his hand on Luke's shoulder, but in the next his hand's nowhere in sight. (00:44:40)
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 Luke, Han, Leia and Chewbacca are going into the Millennium Falcon to escape from the Death Star, a storm trooper is shot and falls twice into the pit. (01:32:30 - 01:32:36)
Continuity mistake: Right before Luke yells at Han after he fires his blaster in the trash compactor, there's a wideshot where Han is sitting among some garbage and holding his blaster with one hand. In the next shot, he has turned the other way and is standing, holding the blaster with two hands. (01:16:15)
Revealing mistake: When Luke opens up a light sabre for the very first time, you never once see the light reflecting from the metal of C-3PO's head (and he was swinging it quite close to the droid's head).
Continuity mistake: When they are searching for the monster in the garbage compactor, the garbage is shaking, except in one shot.
Continuity mistake: When Luke is about to climb the ladder to get into the X-wing, there's a man fixing the ladder who starts to walk away, and another man walking under the wing and leaving. A shot later, the man by the ladder is gone, and the second guy is metres behind, though now he walks away going straight ahead. (01:43:42)
Deliberate mistake: While Luke hears Ben's voice telling him to let go, Vader says, "The Force is strong with this one," and in the following wide shot as they say Luke's computer is turned off, the dent on C-3PO's head is on his right side instead of his left, revealing a flipped shot. (01:55:50)
Deliberate mistake: During the scenes of the Star Destroyer attacking Tantive IV, and also when the stormtroopers board the ship, the Alderaanian consular security force are wearing helmets with an attached communications device at their left, but in a few flipped shots the devices are at their right side of the helmets. (00:02:35)
Continuity mistake: In the beginning of the movie, aboard Blockade runner, a big smudge of dirt appears on top of R2-D2, close to his square-framed lens 'eye'. This was not present a minute earlier and when the droids have landed on the desert, the smudge is gone again. (00:02:40 - 00:09:50)
Other mistake: Before Uncle Owen yells for Luke, if you look very closely, a fly can be seen on the lens (best seen on the special edition release). The fly was removed in the 2019 release.
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.
Other mistake: After they have escaped from the Death Star, Darth Vader says to an officer that they jumped into hyperspace but when you see Han and Leia speaking in the cockpit of the Millennium Falcon, you can't see that blue colour of the sky distinctive to hyperspace.
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.
Continuity mistake: Widescreen version: In the trash compactor, before the scene where the droids leave the control room, both ends of the pole used to stop the compactor are coming into contact with the walls. After the droids leave the room, the right end of the pole is not coming into contact with the wall. (01:18:50 - 01:19:45)
Continuity mistake: When Luke sees Red Leader hit the Death Star surface, an explosion appears out of nowhere and the X-Wing is not seen at all. Furthermore, the plummeting ship would have spread debris over a much wider area considering it was falling at such a shallow angle. (01:48:30)
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