I noticed this while watching the film the other day, and is either an easy or difficult fix, depending on your preference. After Vader slices off Luke's hand, in a low angle shot on Luke, you can see Vader's lightsaber is still ignited, but in all subsequent shots, it has already been turned off. There is no SFX of the saber being switched off, either. The film cuts like this: shot one (seen below) cuts to a medium shot of Vader (with the saber out of frame), and then, to the second shot below where the saber is off. So, you could potentially add in the SFX when it is out of frame, but that cutaway is one second long, at best. So, I'm not sure how good it would play. The alternative is painting out the saber in three different shots following Luke's hand being cut off.

-NJM