When Luke watched the Imperial fleet destroy the rebel starships, Emperor Palpatine could feel the anger and hatred grow inside Luke.
He taunted Luke.
Good. I can feel your anger. I am defenseless. Take your weapon! Strike me down with all your hatred and your journey to the Dark Side will be complete!
Luke pulls his lightsaber to him, activates it, and swings towards the Emperor, only to be blocked by Darth Vader.
Yet from this still image it looks like Luke's lightsaber was closer to the Emperor than Vader's. And Vader's is closer to the camera.
Which means Vader's lightsaber was on the wrong side to block Luke. Luke's lightsaber should have sliced off Palpatine's neck before hitting Vader's lightsaber.
If there is an out-of-universe explanation, I will accept that. Was that scene filmed wrong? Was Mark Hamill a little too fast for David Prowse to swing his lightsaber prop to block? Was it a simple editing oversight? Did the stunt choregrapher make an error?
If there is an in-universe explanation, I will accept that too. Did the novelization say something about that moment? (Perhaps that Luke realized he could not just strike down the Emperor because by the time his lightsaber got close to the Emperor, Darth Vader would have killed Luke. Therefore Luke had to kill Vader first.)