A couple of times in my career have I had a Business Class seat in BA and been issued at the gate with a new seat in economy. The first time the CC sorted it out: the second time (at Nice) I was bumped on to a following flight. Apparently some African VIP diplomat with family appeared, who had an onward connection from LHR and whose connecting flight into Nice was late. Needless to say, I wasn't happy, but it was before the current regulations appeared so I didn't get a lot of compensation from BA. They did provide free telephone calls......As I had ordered a special meal, there was a problem....These days, it would have cost them more than the ticket!
I was flying back once with American when the CC moved the woman with 4 kids from the back row to the row I was in so there was somewhere extra to put trays of glasses. When I asked her if it was new SOP to put young children next to men unrelated to them, I got a dirty look and the family moved back......
But these were all cases where CC did what they could, albeit, in the case of AA, unwillingly. Which generally seems to be the case these days with AA cabin crew. Hardly surprising when one considers the way they have been messed about in terms of employment T & Cs while the big boys at the top get large bonuses.