Having a contract drafted for weekend submission has a few benefits; first and foremost it makes sure that both parties are on the same page as to the expectations, limitations, and concessions that have been made. In addition to that, having a boy read through the contract before signing it at the start of the weekend can be a huge mindfuck as he's being very blatantly reminded of what he's about to endure. I was under the impression that most contracts (particularly regarding rape) may not be enforceable, but stumbled upon this article on some rather draconian aspects of modern rape prosecution. At any rate, if a scene goes so badly that a sub is seeking prosecution under the law there's probably some serious ineptitude on the Dom's part, but it still never hurts to have some modicum of protection in the event a sub isn't mature enough to accept responsibility for mistakenly requesting what he did.
That being said, I recently went hunting for a contract that would be suitable and stumbled upon one that had some language I really liked. Essentially, rather than just detailing limits and expectations it goes so far as to itemize a number of things that might be prosecuted under law as items for which consent is expressly waived. This particular contract goes far enough to even detail allowing castration or similar mutilation, but the important part of the contract is more the language leading into the itemization. As an added safeguard, it also appoints the Dom as "attorney-in-fact" in the event of a medical emergency where the sub may be incapacitated. It's rather long so I'm including it after the jump, but it should be fairly easy to modify for any scene.