The one time I've ever got involved in something like this (I usually keep my nose out)...
I was on an XC service Nottingham-Cardiff. At Nottingham a 2-car 170 came in, and they bolted a second set onto the back. On this route, the train reverses at Derby, so the new unit (without reservations) was now at the front. My sister and I are sat in the small area at the very front (in the end that's not first class).
At Birmingham the train gets rather full rather quickly. A group of 5 people come up to us demanding we move, as they have reservations. I tried to explain that we weren't going to move, as these seats aren't reserved. I offer that their reserved seats will be in the rear half of the train, as there are two units together - hence two coach As. That wasn't good enough for them though. They were adamant we were sat in their reserved seats, as they were at the rear of the train on the way up, and so should be at the front on their way back.
They ended up standing in the vestibule area, and I overheard them (probably purposefully loud enough for me to hear) saying how ridiculous it was they paid all that money for a seat reservation, and that we wouldn't move for them. Another traveller piped up saying they once paid £19 for a seat reservation, and nobody would move. They agreed and said the joke would be on us when the guard came through (who was in the rear unit on non-corridor stock...!)
After Gloucester, the train emptied out enough for them to sit at the table adjacent to us. Rather snarkily, one of the group piped up with "Would you like to see our reservations then?" I said yes, and the look on their face was rather priceless when I pointed out the big fat £0.00 on their reservation coupon