“TCP implementations should follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others.”
Jon Postel, RFC 761, DoD Standard Transmission Control Protocol (1980) · Section 2.10, Robustness Principle · read it
Kept in the 1980 wording. The principle built the internet and is now argued over, because being liberal in what you accept lets errors live forever. Both the rule and the argument against it are worth holding at once.