Summary: I was trapped in a Bone Cage as Diablo moved from Phase 1 to Phase 2 and in the Realm of Terror I was permanently stunned.
Description: Playing by myself, on my Wizard, in Normal difficulty, I had an Arcane Hydra DPSing Diablo. Diablo captured me in a bone cage and the Hydra pushed Diablo over his health threshold pushing him into Phase 2 (Realm of Terror) while I was still in a bone cage.
He never picked me up and roared at me. Instead, he did his stomp and his speech and I moved into the Realm of Terror phase. During his stomp and speech, I was stuck in the Bone Cage. Then, when I appeared in the Realm of Terror, the shadow Diablo did his speech. I was no longer in the cage, but I was stunned during his entire speech.
Then, after the Shadow Diablo did his speech, I was still stunned (hunched over and unable to take any action). I was stunned for at least 5-7 seconds after the Shadow Diablo did his speech (when I would normally regain control). I was then, obviously, killed.
The entire time I was stunned I was attempting to use all sorts of spells, trying to use a potion, and trying to move but I was not able to do anything.
Repeatable: I have not tried to repeat it, but I bet that this is a pretty rare set of circumstances and I bet that it would happen again if the circumstances were repeated.