Happens regularly enough that they're likely to be fine with basket-sitting for a short period. I remember someone in front of the queue at Sainsbury's discovering they were walletless after a whole trolley full had gone through the checkout. They were able to park the trolley out of the way, and fudge the transaction so they could come back and pay at the customer services desk without putting everything through the till again, which seemed eminently sensible.
There's a not-uncommon Aldi/Lidl variant where someone discovers they don't take credit cards, and has to duck out to the cash machine.