[caf]

Quite apart from security, Postel's Principle can hurt the capacity to make backwards-compatible feature additions in the future.

For example, if you have a set of unused flag bits documented as "reserved, must be zero", then a receiver that silently ignores non-zero bits allows senders that erroneously set those bits to propagate. This is fine, until one day in a future standard you want to define new behaviour for one of those bits, and find you can't - because there's large numbers of senders out there that erroneously set it but don't have any idea about the new behaviour.