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)
Continuity mistake: When Luke is climbing to the X-Wing, R2D2 is being brought over right next to where a guy is. He touches the robot and brings it close to him. Shot changes and the robot is away and it's being moved towards the man again. (01:43:46)
Continuity mistake: While Luke climbs the ladder to get into the X-wing's cockpit he asks R2 if the little droid is okay, then as Luke steps up he's holding his gloves in his right hand, but it cuts to the next shot as Luke leaps into the cockpit and the gloves are now suddenly grasped in his left hand. (01:43:50)
Continuity mistake: When the Gold Leader says to the Red Leader, "Red Leader, this is Gold Leader," at the start of the Battle of Yavin, the two Y-Wing fighters outside his cockpit suddenly disappear. This was fixed in the 2004 version. (01:46:05)
Continuity mistake: Towards the end of the film, the Death Star rounds the planet towards the moon in order to destroy it. The movie continually reverts back to the Death Star where a commander informs us of the time until the Death Star is in range of the rebel base. It will then show the panel with the Death Star's co-ordinates and how long until contact. However, it does not count down in minutes, it counts down in seconds. E.g the commander will say "40 minutes till contact" yet the screen will start at 40 seconds and count down, 39, 38, 37. Regardless of whether or not they use the same time system as us, the time would have run out far too quickly. (01:47:30)
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)
Continuity mistake: Darth Vader fires at Luke's X-wing and blows R2D2's top off. Half a second later it shows no damage at all. It's not even scorched. When Luke lands, R2D2's head is destroyed and it's scorched all over. (01:56:15)
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: When R2-D2 is hiding from the Sand People, he is standing in front of a rock. When he comes out, the rock disappears. 2011 Blu-ray and 2019 4K.
Continuity mistake: DVD and Blu-ray only: When Darth Vader and Obi-Wan are dueling on the Death Star, all shots of Obi-Wan's lightsaber are blue. But in two shots, his lightsaber changes to purple. This was fixed on the 4K release.
Continuity mistake: After Vader says "I have you now", in the first frame of the next shot, the TIE fighter disappears for the first frame and reappears in the second frame before it's blasted by Han Solo. It was restored in the first frame on the DVD.
Continuity mistake: Sometimes, the red stripes on the wings on the underside and topside of Luke's X-wing change. For example, compare the shot where he brushes his hand along the underside of the wing of the ship prior to the battle, and the shot before he says, "I'm hit, but not bad."
Continuity mistake: In Mos Eisley, when Ben and Luke are interrogated by the stormtroopers, the background changes. At the beginning and at the end of that scene, the street is large and full of people and beasts, but at other times, the street is small.
Continuity mistake: When Darth Vader is talking to Princess Leia, the chain holding his cape keeps changing between over and under his mask.
Continuity mistake: Widescreen version only: When the stormtroopers are searching for hiding Rebels in the blockade runner, Leia pokes her head out from her hiding place. In the next shot, in the lower left hand corner, something white sticks out behind the foremost "pillar" and then disappears out of view.
Continuity mistake: When R2D2 and C3PO land on Tatooine, the escape pod is on the flank of a dune. When the stormtroopers are doing their investigation, the pod is suddenly on more or less flat sand.
Continuity mistake: When C-3PO and R2-D2 are standing next to each other in Luke's garage, the position of R2's head changes between several shots.
Continuity mistake: Several times during the Death Star battle when the Rebel ships are flying down the trench, their position in relation to the chasing TIE Fighters change. For example, sometimes the Imperials are just behind and then nowhere in sight when, next shot, they are back in close firing range.
Continuity mistake: On the journey from the Cantina to the Millennium Falcon the camera pans left to right to follow the actors as they walk first towards and then away from the camera, towards the hangar. At the beginning of the shot R2-D2 is heading in one direction, the droid leaves the frame momentarily and at the end of the pan, trundles into the shot from an impossible angle.
Continuity mistake: During the scenes in the trash compactor, after Han fires his blaster, the burn marks on the door keep changing shape, size and position. In some of the wideshots, they disappear completely.
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