What can Steven Segal teach us about development

While waiting for my virtual pc to start up, I thought about a quote a colleague of mine mentioned:
Assumption is the mother of all fuck up's - I cannot agree more!
In development there are a lot of assumptions to handle. How should you handle assumptions?

After reading an excellent summary of experiences with test-driven development, I find Fitnesse more and more interesting, or any other frameworks for writing acceptance tests, e.g. Cucumber. And Mike Cohn talks about acceptance-test-driven development in his book "Succeeding with Agile", as a primary role in defining what a system shall do.

So instead of making assumptions, why not help the customer to define a test you can implement as an acceptance-test. Then the there is nothing to assume any more.

The best intro to Fitnesse I have found, is Brett Shuchert's tutorials.