at the end of july i sent a 40-50fter a couple times and last jump i did i jumped simultaneously with a friend and didn't pay too much attention to my posture and landed angled (butt out) as soon as my little bumbum touched the water i felt something went horribly wrong and i had badly bruised my coccyx, now almost 2 months later it still hurts when i have to sit in the same position for longer than 15-30 minutes .... (long drives in the car are hell! )
but seriously i'd say from around 30ft you can seriously injure yourself if you do stupid stuff (you don't want to bellyflop) and from 30 upwards you seriously need to make sure you know how to land properly, as you see i wrecked my ass on a 40-50fter just by holding it out a little bit .....