I flew STL-LAS a couple months ago and a mother and son duo tried to pull this.

I was flying in F. They boarded when their group was called, and were directly behind me as we entered the aircraft. I took my seat, the last F seat in the cabin, then the the mother told her son to quickly slip into the seats across the aisle from me.

They got preflight drinks, but once the FA got the manifest, she must have realized something was wrong, because there were only two pax assigned to the F cabin. She asked to see the scamsters' boarding passes; they claimed they didn't know there was assigned seating (they knew enough to read their group numbers, but they couldn't find seat assignments? I don't think so. Besides, why wouldn't have all those 50 people who boarded ahead of them taken the big seats up front?).

Anyway, I'm relatively young, so I don't know if they thought I was scamming a seat and they'd jump on the train, or what. But, they were sent back to their assigned seats.

d