I was going down the stairs at school after it had been raining. As one might expect, the stairs were wet. I slipped, my head went backward and down, and whacked into the step behind me. Somehow, despite having hit the corner at full falling force, I managed to neither get a concussion nor break the skin. Still hurt, though.
So if I ever say it's not possible to grab the rail once you've slipped, it's because I have personal experience in the matter.