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: When Luke is in Obi-Wan's home, C-3PO tells him "Sir, if you'll not be needing me, I'll close down for a while." and Luke nods. However, when R2-D2 projects Leia's recording, C-3PO turns his head to look at it. (This is only just visible). (00:35:06)
Continuity mistake: The Jawa that shoots R2D2 puts his gun in his holster twice. (00:12:40)
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.
Continuity mistake: Viewed from outside, the Millennium Falcon has 4 windows around a round one, but view from inside, there are only three. (00:52:25 - 00:53:55)
Continuity mistake: Moff Tarkin has fourteen little blocks of color that perhaps indicate rank, seven in two rows. The ones on the top are consistently blue, but at least once the ones on the bottom alternate from orange and red to just orange.
Continuity mistake: The ammo belt of Chewbacca appears on him just before the jump into hyperspace.
Continuity mistake: When Luke and Obi-Wan are stopped by the Storm Troopers and Obi-Wan uses his mind trick on one Trooper, there are a couple of other Troopers standing at Obi-Wan's side at first, but when they are cleared to pass, they disappear.
Continuity mistake: When the heros reach Yavin (the Rebel Base) there is a shot of them getting off a cart. Han's holster is on the wrong leg, and Artoo's small lens is on the wrong side. The film was obviously flipped to keep the visual flow of the scenes.
Continuity mistake: When Luke uses the comlink to call up C-3PO to ask how to get out of the detention block, a rack with 3 blasters can be seen in the control room. But right before the stormtroopers enter the control room, there are suddenly 4. (01:14:45 - 01:18:55)
Continuity mistake: Near the end of the Vader/Obi-Wan duel, Luke and Co. run up to the Millennium Falcon. Luke says something along the lines of "Ben?" and it cuts to a shot from roughly behind Luke that shows Vader and Obi-Wan. Look closely: Obi-Wan already has his lightsaber straight up and Vader is going in for the kill. It then cuts to a shot of Obi-Wan glancing over at Luke. Obi-Wan then proceeds to raise his lightsaber (again) for the famous Jedi-disappearing trick as Vader slashes at him (again).
Continuity mistake: In the Cantina, look at the walrus-like alien in a orange space-suit near Luke. He has hairy paws, but when he pushes Luke, he has normal hands with leather gloves and hairy paws again when Obi-Wan cuts his arm.
Continuity mistake: The windows in the cockpit of Darth Vader's ship are arranged differently when viewed from the inside or outside.
Continuity mistake: When Luke is talking to Biggs and Red Leader, R2D2's position jumps as he is being lifted up to the top of the X-Wing. (Widescreen version, Special Edition.) (01:39:15)
Continuity mistake: Sometimes during the scene in the trash compactor, Leia's sleeves move up and down her arms between consecutive shots. This is best seen in the widescreen version.
Continuity mistake: When Chewbacca and C-3PO are playing holographic chess, there's the black and red set of the control room in the Death Star behind the droid whenever there's a closeup on him.
Continuity mistake: When Luke and Leia are stopped in front of the pit, one stormtrooper arrives at the other side and shoots at them. Two others arrive later but in the following shot, there's only two stormtroopers, not three. One has disappeared and there's one dead on the floor that we have never seen dying. After that, the dead one disappears and two stormtroopers arrive again - it's the same shot as the beginning, only from another angle. Then, there's one that gets shot and falls in the pit and the dead stormtrooper reappears on the ground.
Continuity mistake: As C-3PO enters the escape pod and says, "I'm going to regret this", he is right in front of the round opening. In the next shot, he has already begun to walk through the opening. (00:06:35)
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.