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What is a reorg?

A chain reorganization, or “reorg”, occurs when a blockchain produces more than two blocks at the same time, mainly due to a delayed network or a malicious attack. When two or more blocks are mined simultaneously, the chain would be split into multiple branches, temporarily resulting in a duplicate version of a blockchain. In the case of a temporary duplicate blockchain, the longer branch is usually considered the legitimate or canonical chain, and other chain becomes invalid or lost. If a reorg occurs and the branch of blocks is no longer part of the main chain, it is very possible that the transactions on those blocks become unconfirmed and invalid.
Occasionally, non-deep reorgs of 1~2 blocks often occur due to network delay. Deep reorgs, on the other hand, may lead to network failure and vulnerability to attacks.
You can check details of Polygon Network’s reorgs here.