What Is the Risk of a “Double-Spend” Related to the Mempool and Unconfirmed Transactions?

The risk of a double-spend is highest when a transaction is in the mempool but not yet confirmed in a block. A double-spend occurs when a user successfully creates two conflicting transactions using the same funds.

If a recipient accepts payment based on an unconfirmed transaction, the sender could broadcast a second, conflicting transaction with a higher fee. A miner might then include the second transaction, invalidating the first and defrauding the recipient.