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)
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)
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)
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)
Other mistake: When the stormtroopers break into the control room, the stormtrooper on the right of the screen hits his head on the door frame. On the DVD release they've added a thump when he hits it. (01:18:55)
Other mistake: In the scene where Luke and Leia are about to swing across the canyon inside the Death Star, Leia shoots one last time. She hits the bulkhead behind the stormtrooper on the right and he falls forward. The trooper on the left continues firing (as seen in the screencap). (01:25:25)
Other mistake: After Darth Vader kills Ben, there's a shot from the docking bay towards him. Vader's lightsaber is missing its red color. [This has been fixed in the 2004 DVD, but is still valid for VHS prints.] (01:28:20)
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)
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.
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.
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).
Other mistake: In the Cantina, look at the little alien at the bar, the one which makes mice sounds. Each times he drinks, he doesn't touch his mouth, but his chin with his glass.
Other mistake: In the beginning of the movie, when Leia is uploading a message into R2, we can see C3PO looking for R2. When they show C3PO, there are rebel soldiers behind him, running through the corridor, being shot at and falling down. If you look closely, you can see a black stripe vertically running down in the middle of the doorway.
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.
Other mistake: Just after Obi Wan kills the two aliens in the Cantina, there are two quick "reaction shots" from both ends of the bar. If you are quick, you can see that they are both shots of the same end of the bar with one shot flipped. You can see the same human with the light blue/dark blue suit on in both shots.
Other mistake: During the chasm crossfire scene, Leia says, "Here they come" and points the blaster upwards as she aims at the stormtroopers above. But the laser moves downwards.
Other mistake: In two close-ups of Han before he jumps down the garbage chute, you can see make-up smeared on his stormtrooper suit, under his chin.
Other mistake: During the attack on the Death Star, in one shot an X-Wing has two blue engines with two pink, while they are all supposed to be pink.
Other mistake: Just before Darth Vader enters the Blockade Runner, there are some kind of tiny white-green lasers in the air. They are sometimes in line with the normal red ones. The stormtroopers and the security guards of Leia are both using red lasers, so there is no reason to see green ones.
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