Chain Reorg

A chain reorganisation, or chain reorg, happens when nodes discard the tip of the chain they were following and adopt a different, heavier branch. It occurs naturally when two miners find blocks at nearly the same time and the network converges on one of them; the transactions in the discarded block usually return to the mempool and get confirmed later. Short one- or two-block reorgs are routine on many chains. Deep reorgs are far more serious, since they can undo transactions people already treated as settled, which is the mechanism behind double-spend attacks. This is why exchanges wait for a number of confirmations before crediting a deposit, and why chains with deterministic finality aim to eliminate reorgs entirely.