Chain reorg

A chain reorg, short for reorganization, happens when a blockchain drops one version of recent blocks in favor of a different, longer or heavier chain, effectively rewriting the last bit of history. It matters to readers because transactions you thought were confirmed can briefly become unconfirmed, which is why exchanges and merchants wait for extra confirmations before treating a payment as final. A common example: two miners find blocks at nearly the same time, the network splits for a moment, then one branch pulls ahead and the last one or two blocks on the losing side get discarded, along with any transactions that only existed there. Deep reorgs are rare on established chains like Bitcoin, but shallow one-block reorgs still occur occasionally, especially during periods of high block-time variance.