I have big feet. Unless I'm some unusually deep stairs, my feet usually don't fit on the risers. I'm used to it. Going up, I'm on the balls of my feet on the noses of the treads and my heels don't touch; going down, I consciously tap my heel into the riser at the back of the stairstep and my toes overhang. I slipped on the stairs at work yesterday, both times heading down the stairs. The first time, I had my hand on the railing and my feet just went zooop! forward and onto my ass I went, my back cracked against the stairs. Owwwwwch. I was creaky and sore.
The second time I slipped was backing down the stairs on the catwalks in the lower bay. The treads are expanded metal, steel plates with a zigzag pattern on the upturned edges of the perforations for traction. As I was backing down these steps (like you back down a ladder) my foot slipped through the open risers and shot forward. Fortunately, I maintained my footing because my leg stopped when my shin hit the edge of the step above the one my foot slipped on.
This was the result.
My reservoir of colorful language was severely depleted.