When Yoda is attacked by Sidious his expression is that of utter surprise. This, in turn, surprises me since he was the one who approached the confrontation, had already seen the use of lightning by Dooku and sees the Emperor very slowly raise his hands with the same gesture.
Not only was he unable to control the lightning like with Dooku before (and after with Sidious himself) which can possibly be explained by the attack from a more experienced user, but his expressions show he was not expecting such a move. How can that be? Is it just a poorly executed scene or is there an explanation behind it?