Continuity mistake: Special Edition DVD. When Luke, Obi-Wan and droids arrive at space bar in Mos Eisley, notice the herder with white jacket followed by a dinosaur-creature on a leash and then the R2 unit. In the next shot the dinosaur and R2 are there, but the herder has disappeared. (00:42:20)
Continuity mistake: When we see Ben, Luke, R2-D2 and C-3PO on the cliff looking at Mos Eisley Spaceport, you can see Luke's speeder behind the two droids, but when we see a close-up of the four characters, the speeder is nowhere to be seen. (00:42:30)
Continuity mistake: When Luke and Obi Wan arrive at the Cantina, watch the Jawas sitting outside the Cantina. They're initially sitting in a shadow, but when Luke and Obi-Wan walk towards them, they're suddenly in broad daylight. (00:42:45)
Continuity mistake: In the cantina, the alien who eventually gets his arm cut off is hassling Luke. In one shot he puts his hand on Luke's shoulder, but in the next his hand's nowhere in sight. (00:44:40)
Continuity mistake: When a confrontional alien thug threatens Luke and Ben interferes, the thug has his hand on Luke's upper arm. In the next shot, his hand is on Luke's shoulder. (00:45:00)
Continuity mistake: In the Cantina, the burly alien threatening Luke pulls out his blaster twice, once in the background when Luke crashes into the table, and once as Ben's lightsaber is heard being ignited. (00:45:05)
Continuity mistake: Special Edition: When Han is talking to Jabba the Hutt, as Jabba says, "Why did you fry poor Greedo?", the man standing to his left is pointing a blaster towards Han. In the next shot, the man is no longer pointing the blaster at him. (00:51:00)
Continuity mistake: Special Edition: When Han is talking to Jabba the Hutt, when Jabba begins to says, "What if everyone who smuggled for me dropped their cargo at the first sign of an Imperial starship?", Chewbacca can be seen walking behind them. In the next shot, he is suddenly walking next to a wall that was nowhere near him in the previous shot. In the following shot, he is back to his original position. (00:51:00)
Continuity mistake: Viewed from outside, the Millennium Falcon has 4 windows around a round one, but view from inside, there are only three. (00:52:25 - 00:53:55)
Continuity mistake: When Luke and Obi-Wan enter Docking Bay 94 in Mos Eisley, as the first shot ends, C-3PO is turned to his left and is standing right behind Obi-Wan on Obi-Wan's left. In the next shot, Threepio is turned to his right, and is standing further away from Obi-Wan, behind him on Obi-Wan's right. (00:52:25)
Continuity mistake: When they are leaving Mos Eisley Han wears gloves, but when he pulls the lever to jump into hyperspace, they disappear. (00:54:15)
Continuity mistake: When Alderaan is first seen through the Death Star's viewscreen/window, only half of the planet is lit up. The next time we see it, about 75% of it is lit up. (00:54:45 - 00:55:50)
Continuity mistake: The first interior shot of the Millennium Falcon's cockpit shows small hanging dice. In every other shot, however, the dice are gone. (00:54:55)
Continuity mistake: When the Imperial officer opens the door in the Death Star and sees Chewbacca and Han dressed in an Imperial stormtrooper in front of him, he is seen lifting his head, and in the next shot when we see a close-up of the officer, he is seen lifting his head a second time. (01:08:40)
Continuity mistake: When Luke uses the comlink to call up C-3PO to ask how to get out of the detention block, a rack with 3 blasters can be seen in the control room. But right before the stormtroopers enter the control room, there are suddenly 4. (01:14:45 - 01:18:55)
Continuity mistake: When Leia shoots a hole in the detention area wall for their escape, the hole is not big enough to jump through, but when they all jump in, the hole is much bigger. (01:15:10)
Continuity mistake: In the Detention Block, Han and Luke are in their stormtrooper outfits and are firing at the enemy. They go to a shot of Han then to Luke. Luke's white shoulder pads of his stormtrooper outfit are clearly missing, and you can see a black undergarment. They then go back to Han and then back to Luke again, where the shoulder pads are now back. (01:15:20)
Continuity mistake: In two shots of Chewbacca in the beginning of the trash compactor sequence, there's a little area of blue paint on the right side of the hatch. For the rest, the blue spot is missing. (01:16:00)
Continuity mistake: Widescreen version: In the trash compactor, as Han says: "What an incredible smell you've discovered." Leia is leaning against some garbage, and then turns her head. In the next shot, she is no longer leaning against the garbage, and her head is turned another way. (01:16:05)
Continuity mistake: When Han is about to fire his blaster at the door of the garbage compactor, Chewie moves away from the door. In the next shot, he is back in his previous position, and moves away from the door again. (01:16:10)
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