Which law in which state in which circumstances? If a knife salesman tells me his knives can't hurt people, and signs a contract saying he'll assume all liabil

Which law in which state in which circumstances? If a knife salesman tells me his knives can't hurt people, and signs a contract saying he'll assume all liability, I am nevertheless liable when I stab someone with it. The judge will say you damn well should've known you couldn't go to sleep in a car you're operating, and are in fact culpable for your negligence. "But Mercedes said!" "If Mercedes told you to jump (or drive) off a bridge..."