Comment by perlgeek

Most financial systems are designed to have room for mistakes.

Banks keep ledgers of transactions instead of just the balances, so that if an account balance is off, it can be recalculated from the transactions.

Transactions can be cancelled in case there was an error.

Invoices are often checked by humans before they are sent to customers, introducing an option to catch errors. If the customer receives a faulty invoice, they can contact whoever wrote it and ask for a correction. And so on.

Of course, there isn't room for error in every corner -- having a flaw in an authentication or authorization protocol could be very costly to recover from.

But in general, the assumption that in finance in general, there is no room for mistakes is simply false.