Postel's law - love it! Just talking with the roommate the other day about why we get along so well. Postel's law is the perfect summary:

TCP implementations should follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others.

http://en.wikipedia.org/wiki/Robustness_Principle

In my own mind I've always named this "Two Way Slack", but never had such a concise formulation before. Thanks!

But I disagree with your comment a bit, at least in the general human case. Slack on both ends tolerates some deviation from Postel's law. You get very robust when both sides observe it, still some robustness without it, but there's still always the potential for failure.