It could be a till error, Derek.
A year or so back I was shopping in a 24-hour Tesco at about 3am. My contactless card was initially rejected at the self-service till (for a bil of just under £30), so I had to use chip-&-pin. I managed to mis-key a digit, so I then had to repeat the process. Even though I was certain that I'd keyed in the correct number at the second attempt, my card was again rejected. With a third time of trying though my card was accepted and I got a receipt. However the till was still showing that I'd not paid!
I made a point of finding an assistant (which isn't always easy at 3am in Tesco) to show her
(a) what I'd got in my bag ;
(b) the receipt for all my stuff ; and
(c) what was still left showing on the till display,
as I didn't want someone to look at that display and think that I'd walked off without paying.
The assistant was totally baffled but she checked the date and time on the receipt and accepted that I must have paid. (She asked if I'd mind if she kept the receipt, as she needed to show it to a manager when he came on duty later on. I said that I didn't have a problem with that, as long as she was happy that I wasn't nicking stuff from their store!).