Continuity mistake: When you first see the Millennium Falcon, it is in the hangar. If you look close enough, you can see that only half of the ship is there. This is because it was too expensive to build a full-scale model.
Continuity mistake: When Han Solo and Luke Skywalker escort Chewbacca up to the Cell Bay to rescue Leia they are still wearing the stolen Stormtrooper uniforms, including white boots. Yet after they escape the trash compactor they are back in the boots they were wearing when they escaped Tatooine. While we can believe the rest of their outfit can fit under a Stormtrooper uniform, the boots wouldn't.
Continuity mistake: When Vader picks up the rebel in a choke grip, the legs of the stormtrooper in the doorway behind them are apart. In the shot of the rebel's feet, the trooper's legs are together. When Vader tosses the dead rebel into the wall, the trooper's legs are apart again.
Continuity mistake: When Luke and his aunt and uncle are eating at the table, the white drink pitcher alternates from being next to Luke's cup to being across the table throughout the scene.
Continuity mistake: When Aunt Beru speaks to Luke about buying droids, there are many shrubs besides Luke. In the next shot, they are gone.
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: 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: 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 R2D2 has finished showing Leia's holo-message to Luke, C3P0 says something like "he has become a bit eccentric." When he says that, there is a wire coming from his belly. It was not there before or after.
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: 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: 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.
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.
Continuity mistake: After Darth Vader kills Obi-Wan Kenobi, Luke shouts "No!" and then there is a gunfight with stormtroopers. During this, R2-D2 is going up the ramp into the Millennium Falcon. But a few shots later, when Han Solo says "Come on!", you can see R2-D2 at the bottom of the ramp and he's going back into the Falcon for a second time.
Continuity mistake: In the scene where Luke finds the remains of his aunt and uncle, in the shots from behind Luke, the wind is blowing from his left. In the shots facing him, the wind is blowing from his right.
Continuity mistake: During the conference in the Death Star, the back of Tarkin's chair is higher in the wideshots than in the close-ups.
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