Reorg

A reorg (chain reorganization) is an event in which a blockchain node abandons blocks it had accepted because a competing branch of the chain proved stronger, replacing part of its recent history with the new branch. Reorgs are a normal consequence of decentralized block production: when two valid blocks appear at nearly the same height, the network briefly splits, and whichever branch attracts the next block (or more accumulated work or stake) wins — nodes on the losing side reorganize onto it.

Most reorgs are shallow and harmless, replacing one or two blocks; transactions in the discarded blocks usually return to the mempool and confirm again shortly. But depth matters: a transaction that was "confirmed" in a reorged-away block temporarily ceases to exist on the canonical chain, which is exactly the window a double-spender needs. This is why exchanges require multiple confirmations before crediting deposits, scaled to each chain's reorg risk. For example, Ethereum Classic suffered deep reorgs in 2019 and 2020 when attackers rented enough hash power to rewrite dozens of blocks and double-spend against exchanges — feasible because the chain's total mining power was small.

A common misconception is that reorgs mean a chain is broken; occasional shallow reorgs are expected behavior in probabilistic consensus, and modern Ethereum makes them rare and bounded, with finalized blocks (after roughly 15 minutes) unable to reorg at all without validators burning enormous stake. The practical rule for users and businesses is simple: treat recent blocks as provisional and wait for depth or explicit finality proportional to the value at risk.