Happened to me once on an SQ flight back to SIN from (I think) BKK. I was sat in my economy class seat. 43C which is one of a pair and someone had said it was a good seat and worth trying.
It was because after I had been there about 10 mins, a couple came along and one of them was allocated 43C - called over the stewardess and after a bit of a conflab amongst the crew I was whisked off to Business Class. I was quite happy with the solution...