[tptacek]

There are two different functional components of a contract; the "terms" and the "statement of work" (SOW). They're often combined in a single "standard contractor agreement" with a fill-in-the-blanks "SOW exhibit", but just as often you have a distinct master agreement with terms and a SOW for each project.

The SOW is responsible for laying out what work you're going to do. If a changing spec is a project risk, a typical consultant response to that would be to build some of the spec language into the SOW, so that if the spec changes in a way that materially harms your ability to complete the project, you have recourse with the client.

The typical best case scenario for spec slips on projects is a client that extends the contract to account for them, and a somewhat rigorous SOW is a good tool for making that happen.