The Robusness Principle is a good principle for writing network server/client software. If you follow it your software is less likely to fail when interfacing with other software. I also find it to be an optimal heuristic when it comes to interacting with other human beings.
Be conservative in what you do; be liberal in what you accept from others.
-- Jon Postell
I wonder if there is a formal proof that it is an optimally efficient algorithm for interacting entities who don't completely know eachother's context/protocol, from the perspective of information theory?