I had a bit of a weird situation at the end of last year, with regards to seat reservations. I was travelling from Birmingham New Street to Euston after visiting the Christmas market in Birmingham city centre. I took my reserved seat in coach E (I always reserve seat 17 in coach E on Pendolinos when travelling southbound in standard), and nothing out of the ordinary at this point. At Coventry, a couple board and walk over to where I am sitting.
"Excuse me, you're sitting in my seat", the man says to me.
"Sorry, but I have reserved this seat", I reply, showing my ticket and seat reservation.
The man simply repeats the same line as before, and his partner then starts saying about how they reserved the particular seats themselves. They showed me their tickets, but I couldn't see their reservation details (they were Print-At-Home tickets, so I possibly just didn't spot them). I showed them my ticket and reservation again to reiterate that I had reserved this seat from Birmingham New Street to Euston, but they weren't accepting this. Rather than cause a major scene, as people were starting to look in our direction, I just relented and let them have the seat, but left with the parting comment "Looks like the system has double-booked the seat or something". I moved down into coach F, which was actually fairly empty compared to E, so it wasn't so bad that I moved.
I understand that, as I had a reservation for the seat, I was under no obligation to move, but sometimes it's better just to let things go rather than potentially cause issues.
Slightly off-topic from the main discussion, but with this all in mind, is there a possibility that the seat could have been double-booked? Like I say, I didn't see properly their reservation details, so they could have been in the wrong coach or even on the wrong train. However, if everything was as they were saying it was, is it possible that the seat was somehow double-booked?