Revealing mistake: When Wedge leaves the Death Star trench, the green screen is visible behind him for a few frames. (01:54:38)
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: On the Jawas' transporter, when R2D2 looks around in one of the shots, you can very clearly see the actor, Kenny Baker, inside through his eye hole.
Revealing mistake: As Luke prepares the line and grappler to swing across the shaft, Leia provides cover fire. In one close-up, you can see a cartridge case being ejected from the 'blaster gun'.
Revealing mistake: Special Edition. When Han is speaking to Jabba in the hangar, when they are talking about the percentage, or something like that, Han says that 20% is too much. When he says that, his sleeve passes through Jabba's arm.
Revealing mistake: At the start of the Vader vs. Ben lightsaber battle, when Ben turns his on, a cord can be seen that runs up Ben's sleeve.
Revealing mistake: In the Death Star, when Han is screaming and chasing some stormtroopers and enters a room full of them, some are cardboard.
Revealing mistake: On the Death Star when C-3PO makes the announcement that he and R2 should join the others back on the Millennium Falcon, as R2 begins to move, the poor droid's right leg becomes rooted and begins to pull away from its socket.
Revealing mistake: After they free Princess Leia, other stormtroopers make the door explode to enter the detention room. You can see that the door is pre-cut.
Revealing mistake: After Luke orders a drink in the Cantina, there's a shot of two purple-skinned aliens. One of them has big, purple, alien hands, but the other one is wearing white gloves over human hands.
Revealing mistake: When Han shoots a stormtrooper and runs after the rest of them, the stormtrooper he shot moves his leg out of the way so Han won't trip.
Revealing mistake: When Luke and Leia are trying to escape the Death Star and they are stopped in front of the chasm, the stormtroopers shoots on the door, but you can see the pyrotechnic charges and the explosions are not where the laser touches the door.
Revealing mistake: When Darth Vader releases the admiral and he slumps on the desk, the force of the impact makes the whole set wobble.
Revealing mistake: When the Millennium Falcon is attracted to the Death Star, there is a frontal view of it. At the end of that shot, the sky under the ship becomes suddenly bright and you can see the stick used to move it.
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.
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...
Revealing mistake: In the Death Star, when Chewbacca and Han arrive in the hangar, the shadow of Luke and Leia waiting for their signal is visible.
Revealing mistake: Many times during the film, when someone shoots with a blaster, the shot does not come from the gun. A good example is in the detention block of the Death Star. Luke shoots and the laser comes from 10 centimeters over the gun. Special edition.
Revealing mistake: For this one, you will need a letterbox copy that is not the special edition. In the final award scene, there is a long shot of Luke, Han, and Chewie walking through rows and rows of Rebel Troopers. If you look quickly, the troopers on both ends of the screen are cardboard cutouts. They are cropped off in the Pan and Scan version, and digitised in the special edition, so it has to be the right copy to see this mistake.
Revealing mistake: When the stormtroopers stun Leia, if you watch carefully, you can see that she begins to fall before the beam touches her.
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