IDK, my experience is that it can happen not that uncommonly in raids. Not like "WTF healers what are you doing *kick*" but more like... the tank drops and then they go, "Um, healers?"

But then I pull up the death report and point out the tank ate a 60k hit, or took 60k damage during the 3 seconds I was stunned for spellblade holy, or so on. Death reports are great for making this sort of thing go away when it does happen.