How to Use Contracts to Prevent Undefined Behavior

One use case of contracts is to help prevent undefined behavior. What a contract should guarantee is that you don't pass it. The Bloomberg people claim that that's actually an essential use case. And I'm sure about that, but that's another situation where they claim that you never get your contract right the first time in an existing program and therefore you want to use them as a debug mechanism. That seems like a very limited use case but I understand it.