It's happened to me before, on more than one occasion, and usually from the boarding process and onward. I have had UA FAs chasing me down, and outright telling me (and making the incorrect assumption) I am in the wrong section. Now, if some of them would start doing their job, instead of chit-chat with each other, and certain pax (but ignore others), I would gladly show them my ticket.
Some of them are actually surprised I can speak English during the preboarding meal order process.