When I first changed from skis to a snowboard, I took a few lessons. The instructor was teaching me to turn when I hit a large patch of ice on my heel edge. The ice looked like glass, so much so that I could clearly see grass through six inches of it. My board went out from underneath me real quick and I fell backwards hard, whipping the back of my head into the solid ice.
According to the people around me it made a horrible sound, like dropping a bowling ball on concrete. They though I'd cracked my skull open and the instructor was freaking out because it sounded so bad. I was only wearing a beanie at the time but there was no blood or anything.
I think I knocked myself out for a few seconds and I was pretty dazed for a few minutes. I had headaches for the next 24 hours, so I probably ended up with a mild concussion.
So, what did that lesson teach me? Well, I've worn a helmet ever since.