Continuity mistake: When a stormtrooper stands guard by the door of the control room, the door is low enough for someone to hit their head on. But when C-3PO wants to take R2-D2 "down to maintenance", there is plenty of clearance between the trooper's head and the bottom of the door. This has nothing to do with camera angles, as Threepio is the same height as the trooper, and walks out of the room with no problem. (01:22:40)
Other mistake: In the first shot of the Millennium Falcon in the special edition, the top of the ship is missing its radar dish. (00:54:30)
Revealing mistake: When Greedo is threatening Han in the Cantina, there's a black object behind him. When Han shoots him, the object is smaller and some round things appear on the wall. It's visible in the screenshot of the dummy Greedo. (00:50:10)
Continuity mistake: Towards the end of the film, the Death Star rounds the planet towards the moon in order to destroy it. The movie continually reverts back to the Death Star where a commander informs us of the time until the Death Star is in range of the rebel base. It will then show the panel with the Death Star's co-ordinates and how long until contact. However, it does not count down in minutes, it counts down in seconds. E.g the commander will say "40 minutes till contact" yet the screen will start at 40 seconds and count down, 39, 38, 37. Regardless of whether or not they use the same time system as us, the time would have run out far too quickly. (01:47:30)
Revealing mistake: When they are diving into the trash compactor on the Death Star, one can see the metal bars of the grid, that had previously been cut through, wobble like rubber when one of the team brushes against them. (01:18:20)
Continuity mistake: During the battle between Obi-Wan and Darth Vader, there's a scorched slash mark on the wall to Obi-Wan's right before either saber makes contact with it. A few seconds later Darth slashes his saber in exactly that spot (we can see the scorch mark again before he hits it). (01:26:50)
Other mistake: Subtitling and translation can also lead to weird mistakes. Han Solo says (twice in the special edition) that he dumped his cargo because "Even I get boarded sometimes", and in Dutch cinemas the subtitles read as if he said "Even I get bored sometimes." (00:50:35 - 00:53:25)
Continuity mistake: When Darth Vader is talking to Princess Leia, the chain holding his cape keeps changing between over and under his mask.
Revealing mistake: During the final battle at the Death Star, nearly every time Darth Vader is shown in his ship's cockpit, David Prowse's eyes are visible through his helmet's lenses, because the combination of the red lenses in the helmet and the red lighting causes a transparent effect. (01:54:40)
Revealing mistake: After Han Solo runs into the room full of stormtroopers, you can see rubber soles under Chewbacca's feet as he follows him. (01:24:15)
Continuity mistake: Widescreen version only: When the stormtroopers are searching for hiding Rebels in the blockade runner, Leia pokes her head out from her hiding place. In the next shot, in the lower left hand corner, something white sticks out behind the foremost "pillar" and then disappears out of view.
Other mistake: When the Rebel ships are heading towards the Death Star, there is a shot of the squadrons flying towards the camera. Several of the X-Wings seem to be the same model as the lead craft for they sport identical dirt and battle scars. (01:41:00)
Continuity mistake: When R2D2 and C3PO land on Tatooine, the escape pod is on the flank of a dune. When the stormtroopers are doing their investigation, the pod is suddenly on more or less flat sand.
Other mistake: There is a scene where two sandpeople are running to intercept Luke Skywalker after spotting his landspeeder. One of the actors is holding on to his mask in a desperate attempt to keep it on. (1997 version).
Continuity mistake: When C-3PO and R2-D2 are standing next to each other in Luke's garage, the position of R2's head changes between several shots.
Visible crew/equipment: In the original widescreen version, when Luke pulls his landspeeder up to the cantina and jumps out, you can see the microphone boom at the upper left half of the screen. This has been corrected in the DVD release.
Visible crew/equipment: You can see the shadow of the crew on the stormtroopers after Luke and Leia have passed over the Death Star's canyon (the shot where you see Obi-Wan).
Continuity mistake: Several times during the Death Star battle when the Rebel ships are flying down the trench, their position in relation to the chasing TIE Fighters change. For example, sometimes the Imperials are just behind and then nowhere in sight when, next shot, they are back in close firing range.
Continuity mistake: The Jawa that shoots R2D2 puts his gun in his holster twice. (00:12:40)
Continuity mistake: On the journey from the Cantina to the Millennium Falcon the camera pans left to right to follow the actors as they walk first towards and then away from the camera, towards the hangar. At the beginning of the shot R2-D2 is heading in one direction, the droid leaves the frame momentarily and at the end of the pan, trundles into the shot from an impossible angle.
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