they have to be really short stairs for her not to be seriously injured
Not really. It just depends on how fast she was moving when she started to fall down the stairs, or if there are swords somewhere on the stairs that she couldn't grab in time. But otherwise, you can fall down the stair safely. I've done it tons of times when I was a kid with BM where BM went to give me a hug at the top of some stairs and missed, which ended with me going down the stairs.