You know those idiots who will run up and down the stairs at full tilt, and say "if I start falling I can just grab the railing"?
Well, in middle school, I was running down a flight of stairs, at full tilt, after it had just rained. Surprise, surprise, I slipped on the way down, fell backward, and my head smacked into the corner of the step behind me with the full force of my fall. Amazingly, probably because young kids are damn near indestructible, I wasn't knocked out, and I didn't suffer a concussion or even break the skin.
But I did learn a few things from that incident: first and foremost, don't run downhill on a low-traction surface immediately after a heavy downpour. Second, when you say "I can just grab the railing"? No, you can't; you're vastly underestimating the force of gravity.