G&G-Fan said:
That’s another thing that a lot of people miss. When he says “Do not underestimate the powers of the Emperor” that obviously means he faced him before. Obviously he was witness to the Emperor’s power first hand. That actually becomes a plot hole for anyone who removes that duel from the movie, because now how else would he know the Emperor’s power? It fits into the original trilogy perfectly.
I always took the line to mean that Anakin tried to beat the Emperor somehow and failed, thus becoming his thrall. How exactly would require some good writing. Instead Anakin was just dumb and fell for the most basic emotional trickery, then allowed Mace Windu die. And Yoda just knew all of Palpatine’s powers ahead of time, since he saw them from Dooku and wasn’t even surprised by them. They’re now just basic evil powers anyone can learn.