Revealing mistake: When Luke and Leia are trying to pass the pit in the Death Star, Leia shoots at a stormtrooper. The shot passes through him without hurting him.
Revealing mistake: Special Edition Only: In one of the new Mos Eisley scenes, the speeder goes down a road with two Stormtroopers on each side. The troopers are just mirror images of the ones on the other side of the street. Their movements are exactly the same.
Revealing mistake: When Gold Leader enters the Death Star trench, there's a shot from his cockpit. When Luke, Wedge and Biggs enter the trench later, the same shot is used again. This can be seen from a flash of light accompanied by the sound of a shot, followed by three green laserbolts, then another flash/shot and more laserbolts.
Revealing mistake: As Princess Leia is about to undergo interrogation by the droid in her cell, the needle bears the legend "Made In England".
Revealing mistake: On the Death Star, before Luke and Leia try to cross the pit, Luke shoots a panel to close the door. Firstly, he shoots beside it, and secondly, after it explodes, no electronic components are visible in it.
Revealing mistake: During Darth Vader and Ben's lightsaber duel, sometimes you can see a white wire attached to Ben's lightsaber, for example after he says, "If you strike me down, I shall become more powerful than you can possibly imagine."
Revealing mistake: When Ben and Luke are transporting C3P0 after the attack of the Tusken Raiders, the arm of Anthony Daniels, the actor that plays the droid, is visible during the transition to the next shot showing Ben's house.
Revealing mistake: When Luke, Han, and Chewbacca attack the prison of the Death Star, in one shot, the end of the corridor is in a slope while it is supposed to be horizontal. (It looks to me like it's actually a flat panel painted to look like the corridor. The angle looks perfect for the opening shot of them entering the prison floor, it just looks "off" for any other angle.) This is fixed on DVD.
Revealing mistake: When Luke is first practicing with his lightsaber on the Millennium Falcon against the remote, the zapping blasts that it sends at him are in 99% of the case headed nowhere near his body, but disappear in line with it [very poor FX work] and he flinches.
Revealing mistake: When Luke opens up a light sabre for the very first time, you never once see the light reflecting from the metal of C-3PO's head (and he was swinging it quite close to the droid's head).
Revealing mistake: When the Millennium Falcon exits from hyperspace where Alderaan was, a TIE fighter follows them. There is a shot where it passes in front of the Falcon and then disappears.
Revealing mistake: Special Edition: When Luke and Obi-Wan are arriving in Mos Eisley to find a pilot, the Ronto in the background throws the Jawa riding him to the floor. When the Jawa falls to the ground a puff of dust is seen and a thud is heard, but look closely - he never actually hits the ground.
Revealing mistake: Before Darth kills Obi-Wan, he rises his saber to prepare to die and his face and hands are perfectly visible. Right before Darth kills him, there's a wide shot where his face is missing and a black object is used to hold the saber, thus revealing the actor was replaced by an empty cloak to achieve the vanishing effect.
Revealing mistake: In the scene when Luke, Han, and Chewie are in the Detention level, one of the guards walks toward Chewie. Chewie screams and punches the guard. Look carefully and you'll see the guard starts to jump backward before Chewie's hand comes in contact with his body - in fact Chewie completely misses the guard by about a foot.
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