Continuity mistake: When Han, Luke and Leia are trapped in the corridor in the detention block, a loose piece of hair on Leia's forehead keeps disappearing and reappearing between shots.
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.
Deliberate mistake: When Obi Wan deactivates the Death Star's force field, there's an electronic gauge going down and a sign in clear English with the word POWER - as we've seen in shuttles and other places, they don't use English. The studio decided it was better to show "power" and "tractor beam" in English so that the audience would know what Obi-Wan was doing. The lettering we see in Jedi doesn't really affect the plot, so it doesn't need to be English. Still doesn't really fit though. This is fixed on the DVD.
Visible crew/equipment: After the heroes have escaped the trash compactor and we see them arguing outside of it. As they walk away and Han says "No reward is worth this," the shadow of the camera covers his whole face.
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.
Other mistake: In the beginning of the movie, when Leia is uploading a message into R2, we can see C3PO looking for R2. When they show C3PO, there are rebel soldiers behind him, running through the corridor, being shot at and falling down. If you look closely, you can see a black stripe vertically running down in the middle of the doorway.
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.
Other mistake: After the escape from the Death Star, the first TIE apparently blows up, but behind the explosion we can see it still intact.
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.
Revealing mistake: At the beginning of the film, when the Blockade Runner is shot at, some of the lasers seem to pass through the star destroyer.
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.
Visible crew/equipment: After the attack on the Death Star, when the pilots return to the base, the camera crew is visible on C-3PO's body.
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.
Revealing mistake: In the trash compactor, after C-3PO realises that he turned off the comlink, Leia grabs the bar used to stop the compactor and her hand sinks in and puts a dent in the bar. When she loosens her grip, it returns to normal. No wonder it was useless at bracing the walls...
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