Technical reference
Replace-by-fee: an unconfirmed payment is not final settlement
Mempool replacement policy and blockchain consensus are different things.
Research articles and reference entries are published in English. Navigation is available in seven languages.
In this article
Why replacement exists
BIP-125 describes opt-in replace-by-fee signalling. A sender can signal that a transaction may be replaced in a node's mempool, subject to the node's replacement policy. This supports fee adjustment when an unconfirmed transaction waits longer than expected.
The recipient's mistake
A wallet can display a payment before a block includes it. That display is evidence of a proposed transaction, not irreversible receipt. A replacement can spend overlapping inputs and change outputs. Conversely, lack of an opt-in signal should not be treated as a universal promise that no node or miner will accept a conflicting transaction.
Policy is not a protocol guarantee
BIP-125 records a particular replacement policy, not an immutable statement about every current node's configuration. Merchants should distinguish seen, confirmed and sufficiently confirmed states in their own records. A screenshot reading 'sent' does not establish which state the receiving business has actually verified.