"What do you mean I can't have the seat I booked? I BOOKED IT ONLINE?! IT WAS CONFIRMED!!! You a********, you can jam your s***** aircraft up your a***!!"
This is when the check-in staff then ask the pax if they read the terms and conditions, and then show them the conditions for overwing and forward row seat occupation if they didn't.
Bigger problems have been faced at check-in than this.