They'll either shrug their shoulders and build your death car or they simply will assume that maybe you have a very good reason for asking for a death car and it's not their job to question it.
I'm laughing with you, but I've worked with some Eastern European guys who will rip up your spec and then spend 45 minutes telling you are stupid for even wanting such a thing. (And likewise, there's plenty of in-house developers with the "I don't care, I just do what they tell me" blue-collar attitude.)
In addition to very explicit specs and deadlines/milestones, the biggest issue I've seen is that companies assumes they can outsource the entire effort, and grossly underestimate the amount of internal management support required. In most cases, you'll need to match their management structure almost one-to-one; e.g. if the outsourcers have two PMs and an account rep, you will three people on your side in various roles (e.g. PM, business analyst, management sponsor). And once you've added in the needed management overhead, sometimes the cost savings is actually very minimal.