Had a similar incident on Eurostar. Very aggressive woman got on at Ashford claiming I and the other guy next to me were in their seats and we must move. We showed our seat reservations but she insisted they were hers. Spent so long arguing with us that the train departed but when the train manager arrived he pointed out they were on the wrong train....cue smiles all round from the other passengers irritated by their attitude. That was back before HS1 I think and there was a Paris train followed immediately by the Disneyland one and at Ashford they of course occupied both sides of the island platform at the same time. Demands that we stop at Lile for them to change were politely refused.
I seem to have had a day of it yesterday.
First of all, on arrival at King's Cross, woman comes charging up. "Is this the Welwyn train?". Reply with "yes", to which she replies "where is everyone then?". I pointed out that it doesn't depart for another thirty minutes. "You said it was platform 10". I pointed out that she'd boarded the train on platform 11, and her train had just departed from platform 10. She wouldn't have it that it was her mistake, and how on earth it was my error having simply arrived on a train as a passenger and trying to leave the station is beyond me.
Then, in the evening, sitting on train waiting to depart King's Cross, having boarded well early. About 30 seconds before departure time, with the train still being almost empty (only one other in my carriage), an old-ish guy with Brompton storms on, looks at me, huffs and puffs, then starts walking backwards through the train, makes a point of punching the button to open the door to the next carriage. Guessing he had the hump as he didn't get "his" seat? Mental health problems?