When Luke removes his father's mask near the end of Return of the Jedi, he removes just the top part. He leaves the part that goes from Vader's neck to his chin. That part sticks a few inches out from his chin.
You can see the bottom part sticking out from 1:03 to 2:10 in this video snippet.
Now look for his chin piece at the very end of that video snippet; from 2:20 onward. It's missing in the 4-second cut where Luke realizes his father is truly dead and bows his head down in grief.
Where is his chin piece?
It's not visible in some earlier cuts of that scene that focus on Mark Hamill's face because Sebastian Shaw's chin is below the bottom edge of our view. (e.g. - The cuts from 1:47 to 1:50 and from 1:55 to 1:58.) It is visible in other earlier cuts where the camera is far enough back to show more of Sebastian Shaw. (e.g. - The cut from 2:07 to 2:13 shows his chin piece sticking out.) This is likely just a production error, but is there a story behind it?
Did Richard Marquand finish filming that scene after somebody removed a part of Sebastian Shaw's costume? (This video about the behind the scenes making of Darth Vader mentioned Marquand filmed that scene in just a few takes in one day.)
Was the oversight corrected in later releases?