When I was a student, I was getting the train down from Crewe to London, to see dh (then dfiance). I had reserved a seat, because I knew that train was often very busy - and when I got on, someone was sitting in my seat - the forward facing window seat of a group of four - but the forward facing aisle seat was empty, so I went to sit down in it.
The person in my reserved seat told me I couldn't sit in the empty seat, because they had reserved it - there were three of them, travelling together from some management conference, by the looks - and they had clearly decided that if they left one of their reserved seats empty, and used mine, they might not have to share with a stranger.
Anyway - I put my foot down - I'd have had no issue with sitting in the aisle seat, but the fact that the chap wanted to use my seat AND keep his own reserved seat empty, made me see red, so I told him he was in my reserved seat, and I wanted him to move, please!
He did get out, and let me sit in my seat - and from overhearing their conversation during the rest of the journey, I learned what sort of management conference they'd been on - it was an Assertiveness course. I did wonder about suggesting he asked for his money back, since I, a student travelling alone, had clearly managed to be more assertive than him and his friends, without benefit of Management training!