Real world financial systems do have transactions that aren’t fully replicated and hence can be lost as a result. But banks just decide that both the probabilit

Real world financial systems do have transactions that aren’t fully replicated and hence can be lost as a result. But banks just decide that both the probability and sums involved are low enough that they’ll wear the risk. Example: some ATMs are configured that if the bank computer goes down (or the connection to it), they’ll still let customers withdraw limited sums of money in “offline” mode, keeping a local record of the transaction, that will then be uploaded back to the central bank computers when the connection comes back up. But, consider this hypothetical scenario: bank computer has a major outage. ATM goes into offline mode. Customer goes to ATM to withdraw $100. ATM grants withdrawal and records card number, sum and timestamp internally (potentially even in two different forms-on disk and a paper-based backup). Customer leaves building. 30 minutes later, bank computer is still down, when a terrorist attack destroys the building and the ATM inside. The customer got $100 from the bank, but all record of it has been destroyed. Yet, this is such an unlikely scenario, and the sums involved are small enough (in the big picture), that banks don’t do anything to mitigate against it - the mitigation costs more than the expected value of the risk