Happened to me on a LOT flight once. Low T fare, FF number or status weren't on my booking, but the agent said Y was oversold and she had to bump some people.
It was strange....it was an ATR, so the J cabin should have been 1 person per 2 seats...in 12 seats, they had 10 people...so rather than move the curtain and make a smaller J cabin, they gave all 10 of us J food and service, while cramming us in like Y....