Revealing mistake: When Luke is being fed to the Rancor, in Jabba the Hutt's dungeon, there are black outlines around the beast's legs, from the composite's blue screen special effect. This was edited out in the special edition rerelease.

Star Wars: Episode VI - Return of the Jedi (1983)
Ending / spoiler
Directed by: Richard Marquand
Starring: Harrison Ford, Frank Oz, James Earl Jones, Mark Hamill, Alec Guinness, Carrie Fisher, Anthony Daniels, Billy Dee Williams, Ian McDiarmid, Kenny Baker, Peter Mayhew, David Prowse
Genres: Action, Adventure, Fantasy, Sci-fi, Star Wars, Thriller
Luke, Han, Leia, and Chewy arrive on Endor, but they are separated during a skirmish with Storm Troopers. Leia encounters Ewoks, small furry creatures who take her to their tree village. Luke, Han, Chewbacca, C3PO, and R2 become ensnared in an Ewok net, and the Ewoks believe C3PO is a god. The group is reunited with Leia, and Luke persuades the Ewoks to help the Rebels. Sensing Vader is aboard the space station and knowing he can sense his presence on Endor, Luke tells Leia he’s endangering the mission and must leave. He reveals to her Darth Vader is his father and that she is also his child. Luke surrenders to Vader who takes him to Emperor Palpatine on the Death Star. The rebel fleet arrives to attack the Death Star, but the protective shield is still operational. Watching helplessly as the battle unfolds, Luke vows never to turn to the dark side. He struggles to submerge his anger and hate but is goaded by Palpatine to surrender to his emotions and attack him with his light saber. Luke attempts to strike, but Vader intervenes and they duel. Vader hears Luke’s thoughts that Leia is his sister and now he intends to turn her to the dark side. To protect Leia, Luke furiously attacks Vader. He has become powerful and after an intense battle, defeats Vader. Palpatine urges Luke to kill Vader and assume his position, but Luke refuses and tosses aside his light saber. Accepting that Luke can never be turned, the emperor showers him with electrical bolts. As Luke writhes in agony, Vader picks up Palpatine and throws him to his death down a deep reactor shaft. Vader is mortally wounded. Aided by Ewoks, Han and Leia destroy the shield generator. The Rebel fleet attacks the Death Star, delivering it a lethal blow. With destruction imminent, Luke drags the dying Vader to a small ship. Near death, Vader asks Luke to remove his mask so he can look upon his son with his own eyes and then dies. Luke escapes moments before the space station explodes. With Palpatine and Vader dead, the Empire collapses and the Republic is restored. During celebrations on Endor, the ghostly specters of Obi Wan, Yoda, and the youthful Anakin (Vader) appear to Luke. Redeemed and fulfilling his destiny to return balance to the Force, Anakin is a Jedi once again.
Edmund
Darth Vader: Obi-Wan once thought as you do. You don't know the power of the dark side.
Trivia: When they were filming the scenes on Endor, Peter Mayhew was told to stay close to the set so no one would mistake him for Big Foot and attempt to shoot him.
Question: Who is Luke talking about when he covers up his hand and says that he has a promise to keep to an old friend? Who is the old friend?
Answer: Yoda. When he was departing for Cloud City to save his friends in the previous film, he promised Yoda that he would return to Dagobah to complete his training.
OK. But I thought he was talking about the holeish thing in his hand since he covered it up when he said it. I must have misunderstood it.
The hole in Luke's robotic hand reminds him that he got that robotic hand in the first place because he lost his duel with Darth Vader at Cloud City - a confrontation Yoda warned him he was not ready for.
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Suggested correction: This was not a "mistake". The outlines showing around the beast's leg were due to the limited CGI technology at the time the film was made. With advances in special effects in the following decades, the filmmakers were able to enhance the CGI quality in later releases.
raywest ★
But it is a mistake to show that the Rancor is not real, and the outlines show that. It's certainly not intentional.
lionhead
Not sure what you mean that the rancor is not real. Of course it's not real. The issue is, at that time, it was not technically possible to show the beastie without the lines showing. I classify a mistake as something that was not intentional. In this case, it was, due to the limitations of CGI in the 1980s.
raywest ★