Continuity mistake: When Red 10 is destroyed by a TIE fighter during the trench run, inside the cockpit as the craft starts to burn up and explode the pilot and the helmet are completely different to that of Red 10 when seen earlier.
Continuity mistake: When Luke and Han are talking about Princess Leia being executed, C3PO is facing the computer console when Luke says "but they're gonna kill her." In the wider shot C3PO is now facing Han with no time to turn around.
Continuity mistake: The guard that gets shot and falls down the stairs disappears when Han and Chewing back up there to escape the advancing stormtroopers coming out of the lift.
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: At the dinner table, Luke picks up the container in his left hand to pour. A moment later, he picks it up in his right hand. Also, as Owen explains about Obi-Wan, you see Luke's left hand lifting then lowering the glass. When the shot cuts to Luke's face, his right hand is making the motions.
Continuity mistake: The oil streak on Threepio's left, then right, shoulder has totally disappeared when he's seen in the Sand Crawler.
Continuity mistake: During the shoot out on Tatooine the same stormtrooper can be seen falling in the background from 3 different angles as Han boards the Millennium Falcon.
Continuity mistake: When Ben and Luke enter the cockpit of the Millennium Falcon after leaving Tatooine, Ben is seen standing behind Chewbacca on his seat, but in the very next shot he is seen standing by the seat.
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