Next customer took my supermarket points

At the checkout in my local supermarket this morning I had a strange happening. About two thirds of the way through the process and before all my shopping had been scanned, the man behind me gave the Assistant his points card. I didn't notice as I was busy packing and the checkout woman thought he was with me and processed the card. Immediately after she realised that he was the next customer, not with me, and tried to delete the action, without success. I did not make a fuss, the poor woman was so apologetic, I didn't have a very large shop and I think they would have had to cancel the entire transation and start again if I had insisted that I had my points which would have been unfair on the queue of people behind us.

Afterwards it occurred to me that the man who caused the problem didn't seem very bothered which has left me wondering if he does this on a regular basis to add to his own points.

Has anyone else had this happen to them? It makes me think that I had better give my card to the Assistant at the beginning of the transaction and not at the end when I pay, as I usually do.