One of the best flights I've ever had was one evening when an A321 pitched up at LBA to operate the service back to LHR....the very, very few of us who boarded and could sit where ever we wanted, were exposed to relentless service all the way back to LHR. Happy days. Miss British Midland an awful lot.
I had the opposite experience when BMI did a daily Heathrow to Inverness A319. Arrived at Inverness one evening to be told the flight was "full" and I had to sit squashed with all the others. I questioned the load, about 50, so presumed one of BMIs Embraer 145s was substituting. However, an A320 turned up, it was actually one of their mid-haul ones which normally ran to Middle East points, with a huge business class taking up about half the cabin. Nobody was to be seated there, though, and for weight & balance, although we were all in the economy rows behind, nobody allowed in the back rows either, so all squashed together 6-across abeam the wings. Somebody on here suggested it was Inverness dispatch being presented with such an oddball aircraft they didn't know what to do.