I remember hearing on the radio once that someone sat in first class during a weekend where first class upgrades were available. According to them they were going to pay when the guard came round.
However when the guard came round he told them they should have bought the upgrade off him before sitting in first class. There argument was, what is the difference, I'm going to pay whatever happens.
That was a few years ago now I heard that story.
This seems incredibly unusual and against how Weekend First generally works. Sounds like there is a tad more to this story.