Fault Proof

An optimistic rollup's mechanism for proving the sequencer posted incorrect state. Anyone can post a bond and challenge a state assertion; if the challenge succeeds via an interactive proof game on L1, the challenger takes the proposer's stake and the bad state is rejected. OP Mainnet deployed permissionless fault proofs in 2024; Arbitrum's BoLD (Bounded Liquidity Delay) shipped in 2025. The 7-day challenge window is what gives optimistic rollups their delay characteristic.