During the Diablo encounter there is an issue when exiting the Shadow realm portion of the fight and Diablo using the ability "skeleton prison." If Diablo uses this ability and your character gets stuck in the prison and for some reason he pushes over into the 3rd phase simutaneously you become permanently incapacitated for the rest of the encounter. Either until Diablo kills you or your party kills him. This has happened in both the Normal and Nightmare difficulties of this encounter. At this time I have not progressed far enough into Hell to test this as of yet.
Summary - Diablo shadow realm finishing up while he casts shadow prison. Your character becomes permanently incapacitated for the entire duration of the encounter unable to use abilities, potions, etc. Diablo is still able to target you with abilities during the incapacitation of your character. This persists until your party kills Diablo or the encounter is reset.