Salusa, that shouldn't happen once the dorrs have been closed.
However that person might have been a staff passenger, you never know. It's one of the perks of the job, similar story if you work in Boots you get 40% off your purchases. If you know the owner of a restaurant you might get a free meal.
I can understand it can be annoying, but it's just how the world goes!! It's not somethign that happens only in the airline business!!
If an upgrade is agreed by the senior crew member is one thing, if somebody jumps classes trying to cheat the crew (and the airline), it's a totally different story!!