I was checking out food at a supermarket with debit when the card reader crashed. Cashier rebooted it but it complained that I didn't have enough money in the account, which I definitely did. I checked my banking app and it showed as if the transaction had succeeded, but the supermarket couldn't produce a receipt and refused to give me the groceries. When I called the bank they said there's nothing they can do and it would sort itself out in two weeks. The supermarket info desk radio'd that apparently someone else was also having trouble checking out with debit.
That was the day I vowed to always withdraw a month's expenses in advance and fight against full computerization of cash. I can imagine someone getting screwed real good having their food budget stuck in a banking error for half a month unable to buy food!