Not disagreeing with you, but implementation diverges from spec a lot anyway.
I've had to write decoders for things like HTTP, SMTP, SIP (VoIP), and there's so many edge cases and undocumented behavior from different implementations that you have to still support.
I find that it affects text based protocols, a lot more than binary protocols. Like TLS, or RTP, to stick with the examples above, have much less divergence and are much less forgiving to broken (according to spec) implementations.
I've had to write decoders for things like HTTP, SMTP, SIP (VoIP), and there's so many edge cases and undocumented behavior from different implementations that you have to still support.
I find that it affects text based protocols, a lot more than binary protocols. Like TLS, or RTP, to stick with the examples above, have much less divergence and are much less forgiving to broken (according to spec) implementations.