Well-made points by both whimike and eternaltransit ^

I actually was once upgraded at check-in to F by Gulf Air only to then be knocked back to J at the gate. It happened many years ago, and I was travelling on a heavily rebated ticket, issued because I had B2B dealings with GF at the time. As per eternal's comments, it was the sort of episode that on the one hand can in no way warrant a complaint, but which nevertheless created a less than favourable impression.