A friend of mine who i ride bmx with wasn't wearing his helmet and he did a huge superman seat grab over the old 6ft spine at epic skatepark, and he crashed on his head. He went home feeling a bit dizzy but not to bad. The next morning he woke up and went to get something from his fridge and he just collapsed and he stopped breathing. When the ambulance got there his heart had stopped, but they gave him some adrenaline and managed to resuscitate him. He is fine now (3years on) and has R.I.P. written on the back of his helmet as a reminder for him to never ride without it. I have always ridden with a helmet and i have cracked 3 landing on my head, and one time knocking myself out and that time i would be dead for sure because of the speed i hit.