Continuity mistake: When Luke sees Red Leader hit the Death Star surface, an explosion appears out of nowhere and the X-Wing is not seen at all. Furthermore, the plummeting ship would have spread debris over a much wider area considering it was falling at such a shallow angle. (01:48:30)
Revealing mistake: When Darth Vader flies out of the Death Star you see the light falling on the back of the ship change as he clears the bay. Later, when he regains control of his spinning fighter the same shot with the falling shadow is used to show him flying away. (01:49:30)
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)
Deliberate mistake: While Luke hears Ben's voice telling him to let go, Vader says, "The Force is strong with this one," and in the following wide shot as they say Luke's computer is turned off, the dent on C-3PO's head is on his right side instead of his left, revealing a flipped shot. (01:55:50)
Continuity mistake: Darth Vader fires at Luke's X-wing and blows R2D2's top off. Half a second later it shows no damage at all. It's not even scorched. When Luke lands, R2D2's head is destroyed and it's scorched all over. (01:56:15)
Continuity mistake: Just after the scene where R2-D2 is riding shotgun on Luke's X-Wing fighter and is hit by a shot from Darth Vader, there's a shot of C3-PO standing beside Leia in the rebel HQ, and the dent on C3-PO's head is on the right side (it's on the left side throughout the rest of the movie). (01:56:20)
Other mistake: Drewe Henley, who plays Red leader, is misidentified in the credits as "Drewe Hemley." (01:57:50)
Other mistake: Before Uncle Owen yells for Luke, if you look very closely, a fly can be seen on the lens (best seen on the special edition release). The fly was removed in the 2019 release.
Visible crew/equipment: When Luke, Han, Leia, and Chewbacca are in the trash compactor, there is a shot of Chewbacca knocking on the door. If you look to the right side of him, you can see the reflection of the blue stage lights on the metal part of the wall.
Continuity mistake: When R2-D2 is hiding from the Sand People, he is standing in front of a rock. When he comes out, the rock disappears. 2011 Blu-ray and 2019 4K.
Continuity mistake: DVD and Blu-ray only: When Darth Vader and Obi-Wan are dueling on the Death Star, all shots of Obi-Wan's lightsaber are blue. But in two shots, his lightsaber changes to purple. This was fixed on the 4K release.
Visible crew/equipment: At the beginning when R2-D2 is in the escape pod and C-3PO is arguing with him there is a shot of C-3PO from the side and you can see the reflection of the camera in his head.
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.
Visible crew/equipment: In the original widescreen version, during a long shot of a loading bay in the Death Star, you can see a crew member walk just into shot on one side of the screen, stop, look up, and back out again.
Other mistake: During the scene when Obi-Wan is sneaking around the Imperial ship, a group of stormtroopers march by in a close group. One of the troopers is losing his armour and is attempting to hold it on.
Continuity mistake: After Vader says "I have you now", in the first frame of the next shot, the TIE fighter disappears for the first frame and reappears in the second frame before it's blasted by Han Solo. It was restored in the first frame on the DVD.
Continuity mistake: Sometimes, the red stripes on the wings on the underside and topside of Luke's X-wing change. For example, compare the shot where he brushes his hand along the underside of the wing of the ship prior to the battle, and the shot before he says, "I'm hit, but not bad."
Continuity mistake: In Mos Eisley, when Ben and Luke are interrogated by the stormtroopers, the background changes. At the beginning and at the end of that scene, the street is large and full of people and beasts, but at other times, the street is small.
Continuity mistake: When Darth Vader is talking to Princess Leia, the chain holding his cape keeps changing between over and under his mask.
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