If you die inside of Naxx 10 man (And possibly 25 - I haven’t tested it), when you respawn, you are immune to fall damage until you change zones - With a loading screen, that is, log out, or die from other means.
I’m not sure if it has a ton of use, but it does make getting the falling achievement super easy, and it’s fun to run around by the summoning stone, get someone to chase you, and then jump off the platform. I’ve managed to get a few people to follow me down, and die when they land.
hehe its not very usefull, butt very funny when they follow