Postel's Law

Be conservative in what you send, liberal in what you accept.

-- Jon Postel, 1980

Established during the creation of the Internet Protocol, this principle encourages the development of robust software; it’s often referred to as the Robustness Principle. There is a trade-off to robustness, of course: it can increase maintenance costs and compromise security if not implemented properly. This law conflicts to some degree with certain kinds of software interfaces, e.g. event-carried state transfer.