Star Wars Episode 6: Return of the Jedi - How Many Lightsabers Does Luke Have?
At the end when Luke is fighting with Darth Vader:
When Vader is taunting Luke saying all the stuff about him joining the darkside, look in Vader's left hand. You can clearly see that he is holding Luke's lightsaber!
This is not a slip-up. It is a cut-scene where Luke throws his lightsaber over to Lord Vader, refusing to fight. The scene was probably cut becuse they ran out of time or something.
Actually, if I understand it correctly, The scene goes like this: Vader throws his lightsaber at Luke, causing him to fall and drop his lightsaber. Which Vader recovers and you can see if you look closely in any scene right after that where you can see Vader's hands at waist level. Then when Vader says to Luke: "If you will not turn... blah blah blah, maybe your sister will." The scene was actually supposed to be Luke turns the corner, He uses the Force to pull the Lightsaber out of Vader's hand. That's why Luke looks like he's making that kinda reverse lunging motion when he attacks Vader in anger.