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".
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 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.
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.
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.
Visible crew/equipment: When the Blockade Runner is being attacked, just before C-3PO says something like: 'Do you hear that?' to R2-D2, he turns his head and the camera crew is visible on his temple.
Continuity mistake: When R2D2 is captured by the Jawas, a Jawa welds a restraining bolt to the middle of a panel on R2. When the shot changes to a close up, it's being attached to the top of the panel. The shot changes again, and it's welded to the middle again.
Continuity mistake: In the garbage crushing facility, when the walls are coming in, you see them with about 15 feet space, then it zooms to the actors, they talk for about 5 seconds, and then it zooms out again with still 15 feet of space. The walls are coming in for about a minute. At the rate they were moving, they should have been crushed in around 20 seconds.
Other mistake: When Han Solo, Luke, Obi Wan, C3PO and R2-D2 are on their way to the Millennium Falcon for the first time there is a scene where they have to walk down a few stairs. If you look carefully you can see that R2-D2 is struggling to get down the stairs but in the next shot he's down with the others.
Continuity mistake: Just after the creature lets Luke go, Leia says, "It could be worse." In the next shot (after the compactor makes its first sound), Luke can clearly be seen saying, "What's that?", but his question has been edited out.
Continuity mistake: When Luke and R2 are doing the trench run, a tie shoots at Luke and blows the top of R2's head off. But in the next scene R2's top is on again.
Revealing mistake: When the Millennium Falcon exits from hyperspace where Alderaan was, a TIE fighter follows them. There is a shot where it passes in front of the Falcon and then disappears.
Continuity mistake: During the Death Star trench run, Wedge quits because his X-Wing is damaged. The sky behind him changes from grey to black in the same shot.
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