I've had that happen once. Flying back home TPA -> JFK -> BUF with my brother and sister in law back in March. I'm Mosaic, they are not.

Checked Flightaware for our plane, and it's an A321. Found that odd, since TPA-JFK isn't a Mint route. Sure enough, there had been some plane bingo, and we had the goods! They upgraded all 3 of us to the Mint seats, plus one or two other Mosaic members. Wasn't the full on Mint service, but was still nice, especially for them, as they haven't had that experience before.