Continuity mistake: In two shots of Chewbacca in the beginning of the trash compactor sequence, there's a little area of blue paint on the right side of the hatch. For the rest, the blue spot is missing. (01:16:00)
Continuity mistake: Widescreen version: In the trash compactor, in the scene where Han says, "One thing's for sure, we're all going to be a lot thinner," the pole used to stop the compactor is leaning on the wall in different angles between the wide shot and the close-ups.
Other mistake: Drewe Henley, who plays Red leader, is misidentified in the credits as "Drewe Hemley." (01:57:50)
Continuity mistake: As R2-D2 is rolling through the canyon, he's on a smooth part of the land. He then stops and looks around. When he starts moving again, the terrain has changed and is now full of rocks and pebbles. (00:11:20)
Continuity mistake: When R2D2 and C3PO escape from the Blockade runner, the rate at which the escape pod turns when seen from the outside does not correspond with what we see from inside.
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)
Other mistake: Just after Obi Wan kills the two aliens in the Cantina, there are two quick "reaction shots" from both ends of the bar. If you are quick, you can see that they are both shots of the same end of the bar with one shot flipped. You can see the same human with the light blue/dark blue suit on in both shots.
Continuity mistake: When a confrontional alien thug threatens Luke and Ben interferes, the thug has his hand on Luke's upper arm. In the next shot, his hand is on Luke's shoulder. (00:45:00)
Continuity mistake: When Han is about to fire his blaster at the door of the garbage compactor, Chewie moves away from the door. In the next shot, he is back in his previous position, and moves away from the door again. (01:16:10)
Factual error: After the Death Star, when TIE fighters are attacking the Millennium Falcon, each time one explodes at high speed, the debris and the fire does not keep moving.
Continuity mistake: When Han and Leia try to use a pole to stop the trash compactor, Leia says, "Help me." Two shots later, Han can be seen lifting the pole. In the next shot, Leia is suddenly lifting it with him, and Han is holding the pole differently. (01:18:40)
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: After Owen buys C-3PO from the Jawas, Luke says, "All Right. Come on." and R2-D2 begins to turn his head/dome. In the next shot, his head is still turning, but his dome is in a different position from the previous shot, evident from the droid's features. (00:17:45)
Visible crew/equipment: When Luke and Leia are stuck on the bridge, just before the stormtroopers arrive on the opposite platform Luke remembers the cable and sees the thruster like things over the top of him and Leia, when we get a view of them a small shadow bobs up and down at the centre bottom of the screen.
Continuity mistake: After the droids leave the control room of the Death Star, there's a close-up of Luke in the trash compactor, pushing against some garbage. In the next shot, he is pushing against the wall. (01:19:45)
Continuity mistake: When the Millennium Falcon escapes from the Death Star, each time we see Chewbacca, the sky behind him is dark grey with a few stars rather than black and and full of stars. Some shots later, the sky is normal.
Revealing mistake: In the Death Star, when Chewbacca and Han arrive in the hangar, the shadow of Luke and Leia waiting for their signal is visible.
Revealing mistake: Many times during the film, when someone shoots with a blaster, the shot does not come from the gun. A good example is in the detention block of the Death Star. Luke shoots and the laser comes from 10 centimeters over the gun. Special edition.
Continuity mistake: When the garbage compactor is almost completely closed, Chewie holds the walls with his weapon. In the next shot, he is standing by the wall crying and in the other shot, he is holding the walls again.
Revealing mistake: For this one, you will need a letterbox copy that is not the special edition. In the final award scene, there is a long shot of Luke, Han, and Chewie walking through rows and rows of Rebel Troopers. If you look quickly, the troopers on both ends of the screen are cardboard cutouts. They are cropped off in the Pan and Scan version, and digitised in the special edition, so it has to be the right copy to see this mistake.
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