Funny you should ask, not only have a fallen through a staircase, but i've run through and drove through ones too.
1) On a job site, I was walking up stairs, supports from the bottom were rotted out (House was on river bed, flooded up to second story after rain) I got halfway up and the stairs gave way, I fell threw em and almost into the basement, Not too fun
2) Same house, same staircase, we had to take it down since I broke it, So I ran into it. Full tilt, lowered shoulder and blasted through it.
3) Rode a bobcat into a metal emergency escape on a farm house by accident. Luckily it had to come down anyway.