After C-3PO gives himself up outside the bunker entrance in preparation for the Ewok ambush, the commander shouts "Bring those two down here". Both the current and subsequent shots reveal that he could only possibly have know that C-3PO was there as R2-D2 was completely out of view to the Imperial forces.
When Vader is watching the Emperor attack Luke, his helmet is very dusty and what looks like streaks of finger marks can be seen on the helmet. In the next shot, just before Vader picks up the Emperor, his helmet has been cleaned and is shiny again.
As Boba Fett is aiming at him from behind, Luke can be seen throwing a kick at a henchman that misses by a mile and has no effect. While Boba Fett flies by him after being hit by Han immediately afterwards, Luke throws yet another kick at the same henchman, missing again - yet this time the guy throws away his blaster and jumps overboard.
In Star Wars: Episode V - The Empire Strikes Back (1980), just before Han Solo is encased in carbonite, the metal shackles on his wrists are removed, but the strap binding his upper arms is not removed. When released from the carbonite, his upper arms are no longer bound.
Before Leia is shot, the burn mark is visible on her poncho.
When they are approaching the power regulator inside the Death Star , Wedge tells Lando "I'm already on my way out" but he hasn't shot the regulator and hasn't turned around yet - which he does after saying this.
There is no rank in the Imperial Military called "Commander". Commander is a rank only found in the Rebel Alliance.
Following the line in Jabba's droid torture chamber, spoken by EV-9D9: "Splendid! We have been without an interpreter since our master got angry with our last protocol droid and disintegrated him," the camera cuts to a 21B Medic droid which is seen being disintegrated. This is not meant to be the actual protocol droid in question, just a demonstration of what happens to droids who displease Jabba.
Endor is repeatedly referred to as the "Forest Moon" or "Sanctuary Moon" throughout the entire movie, yet in the scene when the Rebel Fleet comes out of hyperspace, you see Endor and the Death Star from a distance, but there appears to be no sign of the planet the Endor moon is orbiting. The novelization explains that the planet has been destroyed "of unknown cataclysm and disappeared into unknown realms."
When Luke and Han are meant to be thrown into the Sarlacc pit, they attack the guards and Luke hits one with his lightsaber. The man reacts as if he has been hit with something hard like a piece of wood, rather than a lightsaber, which would have sliced through him. He may have had clothing with beskar in it, which can block/deflect a lightsaber strike, but which wouldn't necessary absorb the impact.
Admiral Ackbar states that the Death Star is "orbiting" the moon of Endor. However, it is actually geostationary above the shield generator. It could be in geosynchronous orbit, which is still an orbit, but with a frequency equal to the orbited body's rotation.
As Ewoks eat humans, as it's almost presented on Han, it's questionable why Leia was safe from this fate after she entered the village. She forged a friendship with Wicket (the Ewok who found her knocked out), so that would have gotten her an "in" with the village. As they have a dress in her size, it's clear the Ewoks have had some positive contact with humans or humanoid species in the past.
When Vader throws his saber at Luke who is hiding on the catwalk in the Death Star, Vader's saber blade is coming out of the butt end of the hilt - not the proper blade end.
Right after Lando says, "Fighters coming in," at the beginning of the space battle, you can literally see TIE fighters in the background appearing out nowhere.
(at around 1h 50 mins) You can see Luke and Vader's shadows, but you can also see the shadows of their lightsaber blades, even though the blades give off light and shouldn't have shadows.
In the speeder-bike chase sequence, several shots are flipped so that Luke's black glove changes repeatedly from his right hand (where it should be) to his left hand.
When the shield generator explodes, a piece of shrapnel can be seen exiting the left side of the screen and then reentering because it bounces of the inside of the building the miniature was housed in.
When Luke is looking at Vader after his duel in the throne room, his light saber blade is projecting through the top handrail.
As Han Solo is running out of the control room, just before the explosion. You can see his reflection on the back of the "screen"
When Leia points the cannon at the deck, she runs to Luke who says, "Come on!" but his lips do not move.
When Luke and Leia are on the speeder bikes, Luke says, "You take that one I'll take these two" telling Leia which enemy to attack. When he says this, his lips don't move with the sound at all.
When Han is looking at the Millennium Falcon and wondering if he will ever see it again, Leia says, "Come on, General. Let's move." However, her lips say, "Come on, Cap. Let's move."
When Leia offers Wicket something to eat, as he approaches Leia, you can see her smiling and moving her lips saying something, however, the only thing you hear her say is, "Come on," in a serious tone.
When Han is approaching Endor, he tells Chewbacca to "fly casual". The words are heard before he starts speaking.
During the speederbike chase, there are several shots where the support arms are visible holding the front of the bikes up.
During the climactic light saber battle, when Darth Vader is searching for Luke in the lower section of the Emperor's throne room, you clearly see that all large structures in the area are suspended from the ceiling with a few support 'poles' holding up the platform that the Emperor's throne sits on. Luke seems to be hiding behind a wall that doesn't exist in the established geography/architecture of the sub-level. When Darth Vader taunts Luke enough to get him to attack, Luke jumps out and the wall is nowhere to be seen as it's just the open staircase behind him.
When Darth Vader died, he should have disappeared like Obi Wan and Yoda did. Since he had been saved and come back to the light side of the force, his fate should have been the same theirs. One could argue that his suit would not disappear with him because it was a part of his evil self and that might be true, but his face can be clearly seen through the flames in the funeral pyre.
When the Emperor first arrives he says to Darth Vader, "in time he will seek you out", Vader doesn't respond. But in the Emperor's throne room he says it again and Darth Vader says, in a surprised tone, "HE will come to ME?"
The Rebel strike team go all out of the way wearing forest green camouflage robes, though weirdly no camo face paint. Even worse, they bring two obvious droids which could have be seen, especially C-3PO who is shiny golden and talks too loudly for a covert mission. The Scout Troopers and Stormtroopers could have spotted C-3PO when he was approaching the bunker twice yet they are known to be dumb.
In the previous movie, Vader told Luke about a plan to overthrow the Emperor and take his place, with Luke as his apprentice. However, in this movie, he does nothing to achieve this goal, until it's necessary to save Luke from being killed. When they are talking about Luke's new light-saber, Vader asks him to join him.
The Rule Of Two states there can be only one Sith apprentice, and Vader, being the current one, should know about this. He shouldn't support the plan to turn Luke into a Sith apprentice, as it would mean him replaced and probably killed.
During the Battle of Endor, Wedge sends a message to pilots in Red Squadron, yet the pilot who responds to the message is clearly an A-Wing pilot with Green Squadron.
During the Death Star dogfight, a TIE interceptor is destroyed by "Grey Leader" of the Y-Wing squadron. However, Wedge ("Red Leader") acknowledges the hit by saying "Good shot, Red 2".
Han thanks Luke for rescuing him, adding that "now I owe you one." Han has already saved Luke twice to Luke's once, so if anything, Luke has paid him back for one and reduced his debt to one.
The dancer that Jabba drops into the Rancor pit partially loses her top as she falls in, exposing her right breast.
After the Ewoks sound the second battle horn, in the rear-view group shot one of them shoots an arrow before the leader gives the order to fire.