Postel's law

As defined by Jon Postel in RFC 793
TCP implementations will follow a general principle of robustness:  be
conservative in what you do, be liberal in what you accept from others.

Postel's law considered harmful

Being conservative in what you do disincentivizes people from
being liberal in what they accept.
Being liberal in what you accept disincentivizes people from being
conservative in what they do.

This is not my idea,

Martin Thompson wrote
A flaw can become entrenched as a de facto standard. Any implementation
of the protocol is required to replicate the aberrant behavior,
or it is not interoperable. This is both a consequence of applying
the robustness principle, and a product of a natural reluctance
to avoid fatal error conditions.  Ensuring interoperability in
this environment is often referred to as aiming to be "bug for
bug compatible".

Postel's Law again
Postel's Law has 2 parts
Robustness Principle Reconsidered