Frequently. I now have to sleep downstairs for my own safety. First time was when I was about three and the dog's tail knocked me down the stone stairs of our maisonette.I recently related that tale on AB :-)
Yes, once as a child when my cousin pushed me and I roly-poly'd down the stairs and about 5 or 6 years ago when I tried to turn round and lost my footing. Fell backwards and scraped my back on the edge of the shoe rack. Got a long scar all up my back now.
Yes and since it was in the Guest house where we lived, they were very long stairs. Hit the gong at the bottom, fortunately we were closed at the time or the guests would have turned up for food. Smashed my glasses and spent ages picking bits of glass from round my eyes. None went into my eyes fortunately.
As a child we once got into serious trouble for sliding down the stairs on a mattress. Does that count?