After the checker finished checking the items through, the terminal tossed an exception and froze ... which required finding the person with the key to clear it. The explanation was that I had "saved too much".
Duh! Unless checkers and accomplices have been scamming the chain, I don't see the point. If they don't want customers "saving" money, they shouldn't run sales. (I used no manufacturers' coupons, only their Shopper's Club card.)