I had an incident a couple years ago SEA-YVR - boarding from the front was delayed while a very large passenger was helped on board (horizon prop plane) so obviously staff knew how big this person was. When I was finally allowed to board she was in my seat and almost filled both- there were no seats left and none for the rest of the day - I wanted to get home so squeezed in for the most uncomfortable 45 minutes of my flying experience. I did contact customer care in a respectful way and was surprised that they admitted that gate agents are supposed to intervene and the customer is definitely supposed to pay for 2 seats.