I think I haven't had any misbehaving fellow pax on AY. The self-upgrade really surprises me as I would expect FAs to have general awareness about pax count in any narrow-body J cabin (and even on wide-bodies they should know). Even more surprising is that the pax was served... However, I would understand if in low J load, a J pax asked for a friend to be let to the front - and in case of a Plat, to have the companion champagne served there. Then of course, if the Y pax isn't so smart, they would return to the back and tell everyone the modified story of J champagne...