I've had this happen to me once, but the person didn't do it on purpose.

I was taking the Island Hopper from Guam to Kwajalein, and it had a few stops on the way. At every stop, they'd make an alternative side of economy deplane and then do a security check. First class did not have to deplane at all. I decided at Pohnpei that I'd deplane and stretch my legs a bit anyway, and when I came back onto the plane there was a woman and her toddler sat in my row. She didn't speak English and it seemed that it was the first time she was ever on a plane - she must have treated it like a bus and just sat down in the first empty seat she saw. I had to get the flight attendant to come over and ask her to move since she didn't understand me. Definitely one of the most interesting flights I've ever been on.