Long ago when I worked in supermarkets, our 12 item line was a hard limit; the scanner stopped working at 12. If you had more than that, it was your hard luck item 13 simply wouldn’t scan.
Many times this caused problems, mainly for the customer.
Customer: Hey it is only 2 more items.
Operator: But I can’t scan it, the scanner stops at 12.
Customer: But it is only two more items!!
Operator: I understand that, but the scanner won’t take them.
Customer: FINE, just start a new transaction!
Operator: If you will please go to the back of the line then.
Customer: WHAT, but I’m here now!!!
Supervisor: What seems to be the problem here?
Operator: More than 12 items.
Customer: I ONLY HAVE 2 EXTRA ITEMS!!!
Supervisor: I understand, if you could please go to the back of the line to get the extra two items, we will be happy to help you.
Customer: WHAT THE FUCK, IT IS JUST TWO EXTRA ITEMS!!!
Supervisor: If you want, them in one transaction we can cancel this one and move you to a full sized checkout.
Customer: …ENRAGED RANTING…
Supervisor: If you are going to be abusive to me or my staff, I’ll have to ask you to leave.
Word of that type of thing gets around, for the number of people through the supermarket, the total number of incidents was very low. But they happened at least once a week.