Upon opening a testing world a thunderstorm began. Upon setting weather to clear and watching the undead burn and bounce around oddly, I set my difficulty to peaceful. One skeleton refused to leave. I was able to punch him once before he became completely unresponsive. I tried cycling back through difficulties and even using a command instead, but the skeleton remained. I presume that he is a graphical bug and that reloading the game will cause him to vanish as he was intended to. Here is an image of my phantasmal friend.