Well, you can't break an Unbreakable Vow....""I'd worked that much out for myself, funnily enough. What happens if you break it, then?""You die," said Ron simply. "Fred and George tried to get me to make one when I was about five. I nearly did too, I was holding hands with Fred and everything when Dad found us. He went mental," said Ron, with a reminiscent gleam in his eyes. "Only time I've ever seen Dad as angry as Mum. Fred reckons his left buttock has never been the same since."“Yeah, well, passing over Fred’s left buttock —”"I beg your pardon?" said Fred's voice as the twins entered the kitchen.
J.K. Rowling, Harry Potter and the Half-Blood Prince
fred, george, harry-potter, ron, vow