I am always insanely nervous that I am going to trip down the stairs. I have never actually taken a spill down stairs (quite a few trips up the stairs though - is that common?), but I am perpetually petrified of it.
I often start to trip and let of a terrified screech just to find myself entirely upright still.
Knock on wood (but not the stairs) that I will never actually find myself head-first at the bottom of a flight of stairs.
PS - those winding staircases are the SCARIEST of all!