As an April Fool's joke, a British retailer that creates online games inserted an "immortal soul" clause ("you agree to grant Us a non transferable option to claim, for now and for ever more, your immortal soul") into their online EULA just to show that no one reads these contract terms. They were right, because apparently 88% people agreed to potentially give up their soul.
Fortunately, the company stated that they will not be enforcing ownership rights over anyone's soul and will be emailing customers to nullify any claim on their souls.
Although, it could be argued that soul-snatching is somewhat unconscionable and therefore unenforceable, right? :)