That reminds me of a time when I was the first person on board an MD80, sitting in F.
This old man wobbles in and sits down in the first available seat, 1B I believe. I questioned whether or not he belonged there, he didn't look like an avid traveler. Of course a few moments passed and the person's who belonged to that seat showed up and he got escorted back to his coach seat.
Hard to say if he was trying to upgrade himself or if he honestly didn't know what he was doing.
I'm always amused by this, if I were an FA I think I would find myself saying, "Sorry, sir, you don't get this nice big comfortable seat. Let me take you to your small cramped seat in the back of the plane."