Terrifying. Skydiving is already brown your pants scary when you're conscious. Imagine being unconscious while you're falling from 12,500 feet in the air. Think about all the horrible things that could happen. That's what happened to skydiver James Lee. Luckily, miraculously, thankfully, impossibly, he survived the fall.
The Telegraph says that Lee was hit by another diver and the blow rendered him unconscious, "leaving him to hurtle uncontrollably toward the ground". The video above is taken from Lee's helmet cam and you can see him spinning, his body helplessly flailing around. It's scary stuff (I think half the scare factor is the wind noise).
Thankfully, two other skydivers noticed what was happening and hurdled towards Lee. They checked if he was okay (he wasn't), got him to a safe spot and pulled his parachute out. Lee eventually came to while the chute was out and he landed safely (with no memory of anything that happened). Crazy.