IIRC there were some hacks on bitcoin ATMs using this sort of knowledge. The attacker would set up a withdrawal of (IIRC) CAD on the ATM, then transfer the bit

IIRC there were some hacks on bitcoin ATMs using this sort of knowledge. The attacker would set up a withdrawal of (IIRC) CAD on the ATM, then transfer the bitcoin. The ATM would see the transaction, and dispense the cash. However the attacker would immediately, before the next block was generated, send the same BTC to another address which they controlled, with a higher fee. The miner would discard the first one and roll the update into a new block. I may have one or two of the details wrong (is there an explicit way in the bitcoin protocol to mark a transaction as superseding a previous one?), but anyway, like many things, it worked until they got caught.