In a slightly disturbing episode of futurama, Fry (the main character) goes back in time and through a series of disturbing events discovers he is his own grandfather. If you think about the situation, you can go in a perpetual loop trying to figure out the causality. He went back in time because he was born and he was born because he went back in time. In other words, he necessitated his own existence. I thought this was an interesting concept - where the causality is ambiguous. Is this situation observed in physics?