In an Optimistic Rollup, the new state root is published by operator(s) without being checked every time by the Rollup smart contract. Instead, everybody hopes that the state transition is correct. However, if an incorrect state transition is published, other operators or users (who MUST observe what’s going on in the L1 Rollup contract, executing every single transaction) will be able to point to the invalid transaction and revert the incorrect block, slashing malicious operators.
