Interaction design laws
Postel's law
Also called Robustness principle
Be liberal in what you accept and conservative in what you send: accept messy input from people, and give back clear, consistent output.
People type phone numbers with spaces, dates in several formats and names with accents. A product should understand all of them, rather than rejecting anything not in its preferred form.
Accept the variety, tidy it up behind the scenes, and show it back clearly so people can check it.
In practice
A form that accepts '07700 900123', '+44 7700 900123' and '07700900123' and stores them all the same way.
Common mistake
Accepting anything at all without checking it, which leads to bad data. Be flexible about format, strict about meaning.
Where it comes from
Named after Jon Postel, who wrote the principle into early internet standards, including the 1980 specification for TCP.