That you do not tell the person serving you to please wait 2 minutes whilst you get money/another card from your car outside and then just piss off.
I had this twice today, £40 to £50 pounds worth of shopping, Once it has gone through my till I cannot serve anyone else unless I have a supervisor or manager come and void the whole transaction, I am stood there with people waiting to be served, Obviously had to call for another staff member to open another till.
The second one did come back 15 minutes later and was pissed off that they had to queue up again and that I had to scan all the shopping again,
Why do people do this?