Explain the ‘Replace-By-Fee’ (RBF) Mechanism and Its Risk

RBF is a mechanism that allows a user to replace a pending, unconfirmed transaction in the Mempool with a new one that has the same nonce but a significantly higher transaction fee. The original transaction is then dropped by miners in favor of the new, more profitable one.

The primary risk is 'double-spending' if a miner processes both the original and the RBF transaction, though RBF protocols are designed to prevent this.