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)
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.
Deliberate mistake: On Tatooine, after C-3PO tells R2, "No more adventures," he walks off in the opposite direction. When C-3PO reaches the skeletal remains in the sand, and complains that the malfunctioning little twerp tricked him, the closeups of 3PO are flipped shots (also note the position/direction of the skeletal head). (00:10:50)
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.
Revealing mistake: When the droids head towards Tatooine in their escape pod, a rotating imperial ship is shown in the pod's window, getting smaller as the pod moves away. Regrettably, the star field gets smaller at the same rate as the ship, something that would only happen if you were moving away from a photograph, painting etc. (00:06:55)
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: In the opening shot of the movie, sunlight strikes Tatooine from above, but hitting the two moons from the left. (00:02:00)
Other mistake: After the tractor beam pulls the Millennium Falcon into the Death Star, there's a shot of a MSE-6 droid ("mouse droid") at the feet of four stormtroopers. The trooper in the back right, is holding onto his helmet in a desperate attempt to keep it from falling off. (01:03:15)
Revealing mistake: In the shot where Han Solo and Luke jump down the garbage chute, Smoke outlines the end of the corridor, revealing it is a board with the scenery on it. (01:19:00)
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.
Other mistake: 2004 DVD edition. As Jabba and Han finish their conversation, Jabba turns to his left as Han moves toward the Millenium Falcon. At the end of the shot Jabba then exits by continuing to turn to his left; his tail would have made this movement not smooth- Jabba should have had to move it or twist over it or something. Probably comes from the scene being shot with a person in that place and Jabba being added later; the person could certainly have moved like that. (00:53:50)
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...
Audio problem: Widescreen version: When Tarkin attempts to make Leia reveal the location of the Rebel base, he says, "You would prefer another target? A military target? Then name the system." In the next shot, his head is shown from behind, but the motion of his jaw shows that he is still talking, even though nothing is heard. (00:56:00)
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