When I re-watched the scene, I've noticed something ;

In here, some of Palpatine's lightning is actually reaching to Mace Windu, which also makes sense that why Mace screams in pain while no other lightsaber fighters do in the same position against a Force lightning attack.

You can see Mace's face clear, which is in pain.

And now, it's just obvious ;

Palpatine's lightning actually hits Mace and he screams in pain ;

And then, it hits his legs too.

I wanted to make it clear, since there are still Sidious supporters on this site who believes that Sidious wasn't actually trying to kill Mace Windu and he was toying with him. Which is illogical when you see how powerful that Force lightning was, and it could easily kill Mace if not for his last micro-second lightsaber reaction. It even hits him partially and Mace screams in pain because of that.

In ROTJ, Sidious uses various levels of lightning against Luke. This can be seen when he uses on Luke and at the end says "now you will die." Vader intercedes, and the lightning shows Vader's skeleton, like an xray. Sidious didn't use that powerful of a blast on Windu until after Anakin cut off Windu's hands. He didn't have any intention of killing Windu, imo, until that moment.

@redheathen: The question is how does he know Windu will block it at the right time, and how does he know that Mace can resist it to his pain to some degree right before Mace deflects it right back at him.

I'd think possible answers could include:

Sidious knew it was a possible outcome considering his ability to see possible future outcomes, and perhaps he didn't know Mace would withstand a certain amount of voltage. Sidious didn't start with his most powerful lightning and worked up from there. Perhaps another possibility is Mace's power in the Force?

Uhh, Sidious was not actively trying to kill Windu, and still bent his saber. After Windu was disarmed, Sidious did unleash his full power, and Windu's skeleton is seen.

I really don't see the logic behind of this.

@erkan12: It could be argued that Sidious was saving the killing blow for Anakin, as striking down a member of the Jedi Council and one of the most senior Jedi Masters would complete his descent to the dark side. It's not necessarily my view, but I think it's quite a satisfying interpretation.

@Erkan12:

The logic is that if the Lightning did hit Mace before Anakin intervened, it is that Palpatine was not going full out. And, he wasn't. The saber COULD have bent if Palpatine was all out, and Mace would have his own saber cut his face off, and maybe even choke on ozone, because he was choking on ozone. But, credit must be given to Mace, begin hit by a not full out Sidious is still very bad, as shown to Luke. This shows Mace is tough.