Traumatic brain injuries, pretty much impossible to completely eliminate the risk of. These can be permanent and life altering. Land on your butt and roll back fast hitting your head on the wall. Done. Rare, but it happens.
My subthread started with "you need a LOT of bad luck". You can have a lot of bad luck, just like people can have a lot of good luck. It's not impossible but you should not plan your life around it.