Probably when my dog bit into my thumb and managed to put one tooth in through the nail and another through the underside, and wouldn't let to go. Eventually I just decided to pull my thumb out regardless and I was left with a big gash through the underside of my thumb and the entire thumbnail ripped off. Trust me, you never ever want a nail torn out.
Also, when I fell whilst bouncing on a trampoline and my ankle swung down sideways onto the metal rim. Wasn't fractured, surprisingly, but the bruising must have gone right down to the bone because I couldn't walk for a long time, and my entire ankle went purple. Yum.