Transaction malleability

In this attack, the transaction is logically unchanged (in other words, the payer and payee are not changed) but the expression of the transaction is changed, resulting in the hash for that transaction being changed.

In such an attack, one of the transactions becomes double-spent, but the payment is logically unchanged and the payment will complete.

However, it is difficult to confirm your own transactions and the situation must be handled by the operator.

The Mt. Gox company was attacked in this way and became well-known for it.