Mine are probably gonna seem a bit rubbish after reading all the previous ones!
February this year (day before my 17th birthday) I was snowboarding in France. As I was getting off the lift the snowboarder next to me lost their balance and fell in to me. I toppled off a 10m cliff drop and landed on my arm, which bent 90° in the wrong direction. At least I got a ride down the mountain in a snowmobile. XD
The next worst thing was when I was in scouts and a guy with anger problems decided he wanted to drop-kick me in the balls. I actually went 1/2 metre in to the air, hit the ground and then the pain knocked me unconscious momentarily. Wasn't a nice feeling...