You should stop someone if they forget something they bought.
Billy is a cashier with a long line of customers to serve, but his last customer left one of his bags behind by accident after paying.
Billy wants to make sure he keeps the line moving.
Billy shouts to a coworker to chase down the customer who's leaving and bring them the stuff they forgot.
Billy shrugs and continues to ring up customers, assuming the forgetful customer will return for their stuff.
The customer appreciates Billy going out of his way to make sure they get their purchases.
The customer is upset when they get home and realize they forgot some purchases, and calls the store to complain.