The original Usenet comp.mail.pine newsgroup post  by Mark Crispin (father of the IMAP protocol):
This statement is based upon a terrible misunderstand of Postel's
robustness principle. I knew Jon Postel. He was quite unhappy with
how his robustness principle was abused to cover up non-compliant
behavior, and to criticize compliant software.
Jon's principle could perhaps be more accurately stated as "in general,
only a subset of a protocol is actually used in real life. So, you should
be conservative and only generate that subset. However, you should also
be liberal and accept everything that the protocol permits, even if it
appears that nobody will ever use it."
See my potted history of Postel's law: http://ironick.typepad.com/ironick/2005/05/my_history_of_t.h...
That said, it's still unclear how far this extends: the example given is of an unknown error code, which might lead you to think that the requirement is "syntactically well-formed input where you can't 100% determine the semantics." That's a far cry from the way browsers handle malformed HTML. Similarly, you have to apply some judgment concerning what an agent can interpret the meaning of.