It is not right, though often an invalidly issued ticket would be allowed to be used. However the rules are clear, you can be left stranded (and forced to pay full fare to continue the trip); having the ticket issued does not grandfather in any rule violations, and you can even be denied travel after the illegal sector has already been flown. Once again very low probability of this happening, but it can happen and you don't have a legal recourse for being stranded and forced to pay a high fare (depending on what city you happen to be stranded in). Whenever I have a ticket issued which relies on some obscure rule, I always have the issuing agent document the validity of the routing in the PNR and this has stood me in good stead when future questions arise. Of course this strategy only applies if you are in compliance with the rules and not relying on a mistake.