I know that computers aren't infallible. But I also know that traditionally, tills and bank accounts are pretty reliable. Sure, the store might charge you the

I know that computers aren't infallible. But I also know that traditionally, tills and bank accounts are pretty reliable. Sure, the store might charge you the wrong amount if the price label on the shelf and the PC don't match up, because by law the label on the shelf is the source of truth. But other than that? If the till says my purchases add up to £23.45, and after making the purchase my bank account has a balance of £345.67? I don't validate the arithmetic. How much money is in my bank account? Pretty much the amount of money the bank's computer says is in my bank account, modulo any funnyness like pending transactions and cheques that fail to clear. The bank doesn't keep a shoebox of cash in their vault with my name on it for us to reconcile against.