Did I land on an invisible gnome?

My roomie keeps whining at me if I leave water on the floor in the bathroom after I shower. I always figured, meh, it'll dry away. Well, turns out, the world decided to demonstrate the wrongness of my ways, as I slipped in the hallway because I'd stepped in water. No big deal. But here's the thing. I slipped on absolutely flat floor, but I have a bruise right in the middle of my palm, about the size of my little finger's fingerprint. How does that happen?