At the end, when an ewok hits some stormtroopers helmet to make music as if they were drums, we hear metallic sounds. Stormtroopers armor and helmet are clearly made out of plastic, so they can't produce any metallic sound like this.
During the scene in which Luke tells Leia that Vader is his father, he claims he can sense Vader on the planet. Vader's shuttle does not arrive on Endor until the next scene. This is possibly a white lie by Luke to convince Leia that he must go.
After C3PO 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 C3PO was there as R2D2 was completely out of view to the Imperial forces.
When the Imperial troops on Endor have the rebel force captured, C-3PO walks into a clearing and makes himself known to them. The Imperial troop commander orders his men to "Go get those two," referring to C-3PO and R2D2, but only C-3PO was visible to the troops below.
When riding on the skiff, after Luke tells Han he used to live there, Han turns to Luke when he says, "You're going to die here, you know. Convenient." But in the very next shot he is facing away from Luke.
When Chewie is presented to Jabba in Jabba's palace, C-3PO has slime from Jabba on his chest, face and shoulder in one shot and is perfectly clean in the next. (Slime still on C-3PO on Special Edition version.)
Just after landing on Endor, the rebels spot some storm troopers in the forest. You can see a close-up of Luke and Leia. The left side of Leia's helmet has a large round dent in it. During the speeder bike scene there is a shot of the left side of her helmet and there is no dent.
During Luke's message where he offers C3PO and R2D2 to Jabba the Hutt, when the scene switches to Jabba, then pans down to the small creature sitting in front of Jabba, the bubbles in Jabba's pipe and the smoke behind him can be seen floating down instead of up, indicating the scene was played backwards.
As Boba Fett is aiming at him from behind, Luke can be seen throwing a kick at a henchman (with a "WW2 German style" helmet) 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.
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 Wedge's X-Wing enters the center of the Death Star, it is far in front of the Millennium Falcon. Lando instructs Wedge to take out the power regulator on the north. And as scene through the Falcon's cockpit, Wedge is still very far in front. Wedge copies back and says he is already on the way out. In the next scene, Wedge's X-Wing is just barely in front of the Falcon before swooping up to fire at the power regulator.
In the forest battle Chewbacca is with two Ewoks. They appear from behind a tree and Chewbacca's shoulder belt is over his left shoulder. When the scene shifts to them running towards a walker his belt is over his right shoulder.
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.
Despite the fact that Jerjerrod is commander of the Death Star and Piett is an Admiral, their rank insignia of 6 pips indicates that they are merely captains (as Piett was before his promotion in "The Empire Strikes Back". In "Star Wars" Tarkin, and in "Empire", Piett are wearing 12 pips. In all of "Jedi," no Imperial officer is seen wearing rank insignia with more than 6 pips.
In the Sarlacc fight scene after Luke jumps off the plank he grabs the end of it by his right hand with his crown of head under it. In the next scene he is back in a higher position with his crown of head in line with the plank and both his hands grabbing it.
After Luke uncovers Vader's helmet to reveal Anakin underneath, the breathing apparatus was never removed due to being encircled around Anakin's neck. When Anakin dies and leans back on the ramp, from behind you can see the breathing apparatus covering up the lower half of his face. In the very last closeup shot of from behind, the apparatus has disappeared and you can see Anakin's chin clearly.
During the Rancor scene, when we get a full frontal shot of the beast as Luke tries to find a way out, you can see the puppeteer's blue-clothed arm operating the Rancor on its left side (right for the viewer). It isn't that easily noticeable, since it is dark like the surroundings, but its color makes it stand out.
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.
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.
When Luke swings a blow at the Emperor in the Second Death Star, Vader blocks with his light-saber. So his blade should be between Luke's blade and the Emperor. In the 2004 DVD release, the red of Vader's light-saber is clearly visible on top of the blue of Luke's blade, so it looks more like Luke has just defended the Emperor from Vader. However, it could be that the lightsabres are interlocking.
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 Vader is searching for Luke in the Emperor's throne room, he is holding his ignited 'red blade' lightsaber in his right hand, and another unignited lightsaber handle in his left hand. This is because of a deleted scene where Luke throws his lightsaber out to Vader.
When Jabba's sail barge explodes, the heroes ride away in one of the skiffs, while the other is left behind. It looks as if the remaining skiff is being held up by poles at the end of the craft. However, these poles are a decorative part of the fan-like projections on the rear of the skiff, not supports to hold it up.
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.
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 Han throws the piece of equipment at the Imperial Officer (the one that says, "Freeze!") inside the Shield Generator, the officer begins his 'fall' (in actuality, he jumps) over the bars before it even comes close to him. Fixed on Special Edition and DVD version?
In several scenes where the window behind the Emperor's throne is shown, the stars are moving indicating that the Death Star is rotating. However, the shadows cast by the window separators are not moving - they should be since they are from light outside the DS.
During the Endor battle Leia is moving backwards towards the the shield generators door. As she is firing at the storm troopers one of the blaster shots appears to have not been animated, and instead a gas discharge can been seen coming from the blaster.
There is one point in the climactic battle scene where a TIE fighter magically appears in the middle of the Falcon. The is because the old way of adding ships to a scene involved "cutouts." (This was fixed in the Special Edition.)
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.
When Luke and his party are dropped from the Ewoks' net they shortly thereafter have their weapons taken away by the Ewoks. If you look closely you can see the actor's very human fingers sticking through the glove of the Ewok that takes away Han's blaster.
Han's blaster prop was constructed by modifying a Mauser C96 pistol. During the climactic fight scene between the Rebels and stormtroopers on the moon's surface, the weapon's exposed hammer can be seen moving when Han pulls the trigger. Harrison Ford even seems to cock the hammer more than once.
While Luke, Darth and the Emperor are talking on the death star, the depth of field in the stars doesn't match the camera depth. The window in the background of the shots is blurred due to depth but the stars are sharp.
When Boush is holding a thermal detonator there is a quick flash to Boba Fett raising his weapon. This shot is a mirror image as you can see that the weapon is being held left handed and his aiming device on his helmet is on his left side.
The goof items below may give away important plot points.
When, after being hurled by Darth Vader, the Emperor falls into the shaft the same shout is heard when Luke falls into space after stepping off the gantry platform following having his hand cut off by Vader in Star Wars: Episode V - The Empire Strikes Back (1980). (The scream heard by Luke was only inserted into the Special Edition version. It wasn't in the original, and was removed for the DVD release.)
During the Special Edition where young Anakin appears as a force ghost, he is wearing the Jedi robes Obi-Wan wears during Episodes IV, V, and VI. Anakin never wore these style robes before his conversion to Darth Vader, and are not worn by Obi-Wan until Episode IV.
After Vader saved Luke and the two are in the hanger, Luke is holding the fallen Vader. A tear is visible traveling down Luke's right cheek as the camera is zoomed out. In the next shot, is cheek is dry.
Luke removes only Vader's helmet and face mask during the unmasking, clearly he is still wearing the chin section of his mask (visible in every shot) yet right after Vader dies, Luke leans over him and the chin section is now missing.
After Vader throws the Emperor down the shaft, he leans on the side and on the arm that had just had the hand cut off, you can see him bending his wrist, proving that his sleeve was just pulled over his hand.