Vortex ring state has killed more then one of my quad copters.

When you are flying a quad with a lot of power, it is fun to zoom up vertically, cut the throttle, and drop.

Normally, the motors have enough power to arrest the fall before you hit the ground but if you get into VRS no amount of power will save you.

If you realize you are in VRS before it is too late, apply forward thrust and fly out of it.