of my bad holiday experience, was about 3 years ago, when i went to Tenerife. We got to the airport 3 hours before the flight, choose which seats we wanted, which was about 4 rows from the back, as one of us often vist's the toilets during the 4 hour flight. When we got to the departure lougue one of the British Airways ground staff, said you are being moved to different seats as people with children will have your seats instead. We was all shocked as nothing like this has happen to us before. When we got on board, we had the worst seats on the airplane, right at the front behind 1st class, so all we had to look at during the flight was the curtains and was unable to watch the inflight movie as the tv monitor was behind us. I got up the vist the toilet and i saw a young couple with a baby and a toddler sitting in the seat's which we was given first, it really made me angry.
I read in a newspaper that the same thing happen to a woman who had pre booked her seat months in advance, and she was also moved to another seat. So we will never fly with British Airways again.