Deliberate mistake: After the Tusken Raider has attacked Luke, he raises his stick over his head and shouts. The movement of his clothing is extremely unnatural. During the editing, they put the same footage forward, then backward many times, to add emphasis to his movements, but it looks decidedly odd. (00:29:02)
Deliberate mistake: When Obi Wan deactivates the Death Star's force field, there's an electronic gauge going down and a sign in clear English with the word POWER - as we've seen in shuttles and other places, they don't use English. The studio decided it was better to show "power" and "tractor beam" in English so that the audience would know what Obi-Wan was doing. The lettering we see in Jedi doesn't really affect the plot, so it doesn't need to be English. Still doesn't really fit though. This is fixed on the DVD.
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)
Deliberate mistake: During the scenes of the Star Destroyer attacking Tantive IV, and also when the stormtroopers board the ship, the Alderaanian consular security force are wearing helmets with an attached communications device at their left, but in a few flipped shots the devices are at their right side of the helmets. (00:02:35)
Deliberate mistake: During the 4 Tie Fighter attack you can see the blue screen octagon matte leftovers surrounding the Tie's trying to blend into space. However it's not the same type of black, so they glow a bit. They are constantly confused as shields, but they aren't. It's real easy to spot if the contrast and lighting on your TV is a bit high, but I spotted it without that.