Revealing mistake: On Jabba's sail barge, Luke's hand is wounded before it is shot at.
Continuity mistake: When Leia knocks out a scout trooper with a branch after being discovered on Endor, she is turning to the right. In the next shot, she has turned the other way.
Revealing mistake: During the speeder bike chase, the speeders never cast any shadows on the ground.
Revealing mistake: When 3PO and R2 are being assigned to their droidly duties in Jabba's palace, there is a shot of a poor droid having hot irons branding his feet. The irons are never closer than an inch from the droid's feet, and the steam that allegedly comes from the burning is obviously being shot out of holes in the droid's feet.
Revealing mistake: After Wicket tells the rebels where the secret entrance to the base is, there's a shot of spaceships flying in space. An X-Wing passes through the Millennium Falcon and another one through a Calamarian cruiser.
Continuity mistake: In the scene where Leia frees Han from the carbonite, there's a table on the right side of the room. The position and existence of the fruit and other objects lying on and around the table keep changing between shots.
Revealing mistake: Right before Luke leaps from the speeder bike to the scout trooper's speeder, there's a shot showing the front of the speeders. Notice the area surrounding the speeders in this shot. During the rest of the speeder bike chase, the speeders pass through this area several times, even though they are not flying in circles. This can be seen, for example, from the sunlight hitting the ground and the tree on the left side with a bush in front of it. Also, one of the shots where this area is seen is the shot before the speeders pass two more scout troopers, but the background has been flipped.
Continuity mistake: When Jabba pulls Leia's chain in the sail barge, Bib Fortuna is standing on the right side of the screen, a few meters away. He then takes one step forward, but in the next shot he is suddenly standing behind Leia, as he's pushing her against Jabba. He was too far away from them in the previous shot to have reached them that quickly.
Continuity mistake: When they get ambushed in the shield generator complex, an imperial in black appears from the right of the screen in front of the ledge with the railing. Han throws the container in his hands at the imperial. If you notice, the imperial actually begins to jump over the railing before the box even hits him.
Revealing mistake: When C3PO and R2D2 are picked up in the sand, there is no sand falling off them. Obviously, C3PO's legs and R2D2 where just placed behind some sand (and not in it) or they used blue screen in this shot.
Continuity mistake: When Luke and Han are being executed on Tatooine, Han says his eyes are getting better but still bad. Then he looks directly at Luke and says "You're gonna die here." In the very next line of dialog, Han's head is turned a different way. It looks like it was a different shot.
Audio problem: When Lando is inside the Death Star we see a shot of him scraping the side of some metal. Then he says "That was too close." His voice has the same echo applied to it as the sound effects in the narrow metal tunnel. For unknown reasons this mistake wasn't even fixed in the DVD.
Other mistake: During the final confrontation with Vader on the Death Star, Luke bashes a catwalk railing with his light saber several times, but there are no burns, marks, or indentations on it at all. Considering the power of a light saber (Qui Gon melted a steel door in Phantom Menace), some damage should be visible. Only when he hacks off Vader's hand does the light saber slice through the rail.
Other mistake: In the original cinema release, one of the subtitles for Jabba's conversation is incorrectly spelled - 'there will be no bargin'. This was corrected for the video release.
Continuity mistake: After Leia removes her helmet in Jabba's palace, her hair changes on the right side of her head, depending on the camera angle.
Deliberate mistake: The shots of the prisoner skiff during the transport to the Sarlacc Pit are reversed. This can, for example, be seen as Chewie's bandolier is on the wrong side, and that Han's shirt is overlaying the wrong way.
Continuity mistake: As Boba Fett nods to the disguised Leia after the latter turns off the thermal detonator, his costume is reversed due to the negative being flipped. The easiest part of his costume to see this on is his antenna, which is on the left-hand side of his head, whereas normally it would be on his right.
Continuity mistake: In the first shot of the hologram of Endor and the Death Star, the Death Star orbits Endor in an counter-clockwise direction. In the subsequent shots, the Death Star orbits in a clockwise direction.
Revealing mistake: Look carefully at the close-ups of C3PO's waste where the exposed wires are. You can tell it's fake wiring on top of a black suit which is particularly noticeable when the actor inside the costume as his stomach is moving when he has to breathe.
Other mistake: When Vader's shuttle lands on the forest moon of Endor, an Imperial AT-AT walks near the landing pad. There's a strange shadow that keeps appearing and disappearing on one of its leg. Maybe the shadow of the puppeteers?
Answer: Darth: "Obi-Wan once thought as you do. You don't know the power of the dark side. I must obey my master." It's implied that the dark side is intoxicating, once you totally give in to the dark side it has a hold on you, and appeals to morality and rationalization are useless against your lust for its power. (On a symbolic level, the dark side is a metaphor for vice. Darth Vader is an addict and abusive parent. It's actually funny how many scenes still make sense if you replace "the dark side" with "the bottle" or similar).
TonyPH
If we bring the prequels into it, it's one of the criticisms of those films that they only make the question of how much Anakin is a "true believer" more confusing. But it stands to reason that at first Anakin may feel vindicated in his resentment toward the Jedi. Later on, Vader may not feel that as strongly, but by then his anger has turned toward himself for failing to save Padme. He may feel that a man as terrible as he does not deserve to be "rescued" from the dark side, leading to a feedback loop where he only gets further enamored with its power and does more evil things which causes him to hate himself even more, and so it goes.
TonyPH