I already informed blizzard but thought the community would like to know the following is possible:

Where: When you get caught by the things popping up in the ground in the second stage of the fight where diablo will stun you while he is holding you draining your HP.

What: If he is defeated by other cooperative players while he is holding you, you will remain stunned for the entire third stage with 1 HP and a red screen. When he is eventually defeated in the third stage you gain all your HP back and are no longer stunned.