Replay Attack

In the event of a hard fork occurring without an overwhelming majority agreeing to it (e.g. with a 70%/30% ratio instead of 95%/5%), the blockchain will inevitably be split into two, giving rise to two separate ledgers, say Y and Z, each with different rules for accepting blocks. As a consequence, all valid UTXOs in the common ledger would be valid on both chains, and initially any transaction broadcast to the network could result in coins being moved in both ledgers. If a user A sends coins on chain Y from UTXOs prior to the chain split to user B, user B could easily replay the transaction on chain Z, thus “stealing” A’s coins on that chain.