Chain reorganization (reorg)

Nodes always follow the valid chain with the most accumulated work, or on proof-of-stake networks the branch chosen by the fork-choice rule. If a competing branch overtakes the one a node was following, the node rolls back the blocks unique to the old branch, returns their transactions to the mempool and applies the new blocks. A one-block reorg is a routine side effect of two blocks being found at once.

Deep reorgs are a different matter. On smaller proof-of-work chains such as Ethereum Classic and Bitcoin Gold, attackers renting hashrate have rewritten dozens or hundreds of blocks to double-spend deposits at exchanges. That risk is why exchanges require a number of confirmations before crediting deposits, with more confirmations demanded on chains with less hashrate.

Proof-of-stake chains with economic finality make reorgs of finalised blocks practically impossible without massive slashing, but blocks not yet finalised can still be reorganised. Reorgs also matter for MEV, because a block producer could, in theory, try to replace a recent profitable block with its own.