What Is a ‘Rollback’ and Why Is It a Critical Event in Blockchain History?

A rollback, or chain reorganization (reorg), is when a previously confirmed block is removed from the main chain and replaced by a competing block or chain. It is a critical event because it temporarily violates the principle of immutability and can reverse transactions, causing chaos and distrust.

Major rollbacks are usually the result of a consensus failure or a successful attack (like a 51% attack).