Too often what happens is that the receiver is liberal, which results in senders not having to bother to fix violations of whatever spec the parties purport to be following.

As the sending code continues to be developed occasionally new deviations from the actual spec and from whatever it is that current liberal receivers accept creep in, and the receivers get even more liberal to deal with them.

A few years of this and writing a receiver for what should have been a fairly simple and easy to implement format requires huge parsers that handle a bazillion weird cases.