Basically: it's possible that NDE will cause you to lose health while using Tranquility, if: a) your health is above 35%, and b) the incoming damage, in a single blow, would reduce you to 0. This happens, I suspect, because NDE is prioritized over Tranquility, and so fires before the damage can be absorbed.
Testing:I went up against a Mallet Lord wearing very little gear, ~20k health, and stood waiting for his swing, at which point I fired Tranquility. Result: NDE on cooldown, health and spirit reduced to 35% (the amount of health you're restored to after Near-Death). Potentially could be used for an emergency spirit boost, but I don't see the benefits outweighing the cons.
Would be nice to have this hotfixed in the same way BoP was hotfixed (not a complaint).
It's not game breaking, sure, and I'm by no means a top-tier player (getting !@#$ kicked in Act 2 inferno), but it does tend to throw off your ability to predict the course of a fight, especially against packs of hard hitting elites like Dark Beserkers.
edited to clarify.