After Luke cuts off the Wampa's arm he begins to stagger out of the cave into the snow, deactivating his lightsaber as he goes. The sound of the saber deactivating occurs but the saber is still completely active.
In the Dagobah scene, before Luke and R2-D2 have found Yoda (or Yoda found them), Luke plugs something into R2 then says: "Now, all I gotta do is find this Yoda - if he even exists." When he says: "If he even exists.", Luke's mouth doesn't move. The DVD tried to fix this by adding a little more texture every few half-seconds to Luke's mouth, but it is barely visible; they couldn't fix it without ruining the shot.
In the opening scene, Luke is trying to calm his frightened tauntaun (before the wampa attack). As he says, "Hey, what's the matter? You smell something?" the boom microphone can be seen reflected in his goggles.
According to the credits, Luke's gunner in the Snowspeeder is named Dak. However, all other spin-off media (games, books, etc.) spells the name as Dack, which now seems to be official. Later versions of the movie (special edition, DVD release, etc.) do not correct this.
As Han is about to be frozen he is handcuffed, two Ugnaughts seem to fidget with the cuffs but Han is clearly still cuffed in the shot where he is being lowered in to the freezing chamber. When he resurfaces in a frozen state his hands are not cuffed and appears to have "grasped" at the moment he was frozen.
When Luke tries to use the force to take out the spaceship from the water and he can't, he goes behind a tree and puts on his jacket. When Yoda takes out the spaceship the jacket is clearly unzipped, and when Luke goes around the spaceship, it's zipped an tucked
In a shot of Chewbacca, after the rebel soldiers are telling Princess Leia that "the shield doors must be closed", the shield doors can be seen moving soundlessly. In the next shot of Leia and the rebel soldiers, they aren't moving.
Towards the end of Luke's and Vader's duel, their lightsabers switch positions between shots - we see Luke parry against Vader's strike, with Luke's lightsaber pushing upwards against Vader's (towards Vader's body), but in subsequent shots we see Luke's lightsaber pushing downwards.
Han helps Leia in repairing the Falcon on the asteroid. They begin to kiss. When C3PO interrupts, we see a wide angle shot with the couple's arms around each other, but in the previous shot Leia's arms were not around Han's head.
Several times, during the escape from Cloud City, stormtroopers are shot, but no blast marks are left on their armor. In addition, the intenisty of the guns' blasters against the stormtroopers progressively decreases and increases during the episode.
The sounds for lasers and lightsabers change throughout the movie, even with same weapon being used more than once in a scene. A good example is Han's blaster in the asteroid cave, which makes at least two completely different noises when firing.
When Yoda tells Luke to lift the X-wing from the swamp, he is standing on flat ground next to Luke, who is reclined. Luke stands up, looks down at Yoda, and walks away. Yoda then bows his head and raises the X-wing. As it recedes from the camera, Yoda is seen at the far L now standing on a low knoll. When Luke returns, he has to look up at Yoda, but Yoda never moved.
In the first wide shot of the Executor's bridge, there are black-uniformed officers in the left crewpit only. However, when Vader walks down the command walkway, we see the right crewpit, but it seems to be a reversed image of the left. In addition, it appears to be reused at the end of the film when Vader exits the bridge.
When Luke is hanging from the roof of the Wampa's cave, he uses the Force to draw his light saber to him. When viewed from Luke's POV, the light saber is clearly buried in the snow, but when viewed from the other side, it is just as clearly sitting on top of the snow, unburied.
When Vader contacts Ozzel and Piett on the viewscreen after the fleet drops out of hyperspace at Hoth, the lower bands of the officers' rank insignia are erroneously colored dark red rather than blue in the initial shot but correctly colored in the subsequent close-up view.
During the duel on Cloud City, when Luke is sucked through the broken window, he's holding his lightsaber, but then is holding on to the bridge with both hands, however after he climbs up he has the lightsaber back, even though he didn't have a chance to put it anywhere before catching onto the bridge.
In the letterbox version, when the wampa's arm swings into frame to kill Luke's tauntaun, the end of the "glove" can be seen at the bottom left, as well as the top of the head of the crew member wearing it. Afterwards, when he is grabbed by the monster's arm, for a short time you can see the real arm of the person who is wearing the wampa arm.
The colors of the different buttons on Darth Vader's chest plate vary between the movies. However, since time passes between each movie, it is not unthinkable that he has his life support equipment refurbished and upgraded on occasion.
When Darth Vader uses the Force to throw things at Luke, one of them breaks the glass blowing Luke outside. This indicates Cloud City is pressurized. However, when the Millenium Falcon lands earlier, Lando is able to easily walk out the door, indicating there is equal pressure. However Luke and Vader are battling in what is very obviously an interior portion of the city which is far more likely to be pressurized than the exposed outer hatches.
The Snow Speeder being able to destroy the AT after it is tripped is often said to be an error seen as the blasters are useless beforehand against the walkers. But from various sources (including Andrew Darling in the Q&A section in an early edition of the Official Star Wars Magazine) the explanation is that when the walker is tripped the armor is damaged and unable to deflect the power of the blasters. Also the pilots shoot at the much weaker neck area where no protection is offered.
In the beginning of the battle at Hoth, Luke, aboard his snowspeeder, mentions that the armour of the AT-AT's is too strong for their blasters, but after the first walker is downed, a couple of snowspeeders just fire at it a couple of times and the unit explodes.
When the Millennium Falcon is inside the worm and a tremor makes Princess Leia fall on Han Solo she says, "Captain, being held by you isn't quite enough to get me excited", Han Solo is mouthing her line.
In the carbon-freezing room fight, after Luke forces Vader off the platform, Luke steps back after peering over the edge and turns off his light-saber (we hear the sound effect of the saber shutting down). As he walks back from the edge beyond sight of the camera, you can see the tip of the fighting prop saber swing into view momentarily as a stage hand swaps Luke's "fighting prop" (with shaft) for the "shut off" belt prop (without shaft) out of view of the camera.
As Lando, Leia, R2, Chewie, and 3PO start boarding the Falcon to escape Bespin, you can see a blaster mark on the chest of one of the Stormtroopers firing at them from the doorway. Seconds later, Leia shoots the Stormtrooper in the exact spot and he falls down.
During the escape on Cloud City, just after C3PO mentions the Millennium's hyperdrive (while R2 is plugged into the computer system), you see a stormtrooper's shot hit the wall. However, the black blast mark is there before the shot hits it.
After having kicked Vader off the platform in the Carbon-freezing-chamber during the first part of their duel, Luke turns off his Lightsaber and jumps down to continue the fight. But when Luke jumps we can see him land and stand up in the shadows in the bottom-left corner off the screen. Yet we hear the sound of Luke hitting the ground a second later. This was probably an attempt to make Luke's fall sound longer than it actually was. Also we can see some wires wobble after Vader having hit the mattress outside the camera angle.
Imperial officers wear their rank insignia badges on the left. However, when Vader asks Admiral Piett if the Falcon's hyperdrive was deactivated, the shot appears to be flipped - the officers' badges are on their right. This also happens near the end of the movie on Vader's Star Destroyer, the rank insignia of the Imperial officers changes sides repeatedly, indicating that the shots were flipped intermittently. (This has been fixed on the 2004 DVD release.)
In the updated special edition TESB, as Han, Leia, Landa and Chewy are walking to the trap Vader has set, they walk past one of the new "visualisations" with a lift going up and a view of Bespin. Yet in the following shot, when you see Han holding his gun, the original browny white background is back in its place.
During the battle on Hoth when Luke says "Alright I'm coming in" if you look on the black trim on the left side of the cockpit you can see the AT-AT through it. (Visible in the original theatrical version, partially-fixed on the Special-Edition VHS release and subsequent DVD releases to where the trim is less transparent).
As Leia and the others return to rescue Luke, Leia sends Lando to open the top hatch and he exits the cockpit. A shot or two later focuses on Leia at the controls. On the far left side of the screen you can see Lando's blue shirt as he is still standing in the cockpit.
Just before cutting off Luke's hand, Vader cuts through a piece of equipment on the bridge. As the top of the pipe falls off, both cut ends are wrapped in what appears to be duct tape, most likely to hold it in place for multiple takes. When the scene cuts to a new angle, the duct tape is gone.