Continuity mistake: Special Edition: Luke, accompanied by the droids and Obi Wan, is in the speeder heading into Mos Eisley, but just as the shot ends it looks like they're about to run over a few pedestrians... they're heading full speed at a group of three or four people just when the camera angle changes. (00:40:50)
Continuity mistake: When Vader tells Tarkin that Obi-Wan is in the Death Star, as Tarkin says, "The Jedi are extinct." he is standing between two chairs. In the next shot, he is standing behind one of the chairs. The distance between some of the chairs around the conference table changes during the scene as well.
Continuity mistake: Special Edition: When Han is talking to Jabba the Hutt, as Jabba says, "Why did you fry poor Greedo?", the man standing to his left is pointing a blaster towards Han. In the next shot, the man is no longer pointing the blaster at him. (00:51:00)
Continuity mistake: When Vader is strangling the Rebel aboard the blockade runner, the Rebel's hands keep changing position between shots, as he tries in vain to loosen Vader's grip.
Continuity mistake: After Obi-wan saves Luke from the sand people, there is a shot of him sitting on a rock and his white robe is visible, then there is a close-up and he says "I don't seem to remember owning a droid" and his robe is not visible. (00:30:20)
Continuity mistake: In the control room when Obi Wan decides to leave them, the red beam on the radar behind him points down. When it cuts it is suddenly pointing up. It couldn't have moved that fast.
Continuity mistake: When Luke is outside of the farm seaching for R2D2 with his binoculars, C3PO is not visible in them even when Luke turns in front of him.
Continuity mistake: When Luke is eating with his aunt and uncle, he takes a sip of blue milk, and then lowers his cup twice.
Continuity mistake: When Luke and the others are trapped in the detention corridor, there are some black marks on his stormtrooper armour, on his chest, which weren't there before. These marks change slightly between shots, and in the wideshots they are missing.
Continuity mistake: When R2 and 3PO are walking in the desert look at the sun's reflection on C-3PO's head throughout the shots. It keeps changing places.
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: In the opening shot of the movie, sunlight strikes Tatooine from above, but hitting the two moons from the left. (00:02:00)
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.
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)
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)
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)
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: 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)
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