Submitted by alexfilippov314, also found by t0x1c
https://github.com/code-423n4/2024-03-taiko/blob/f58384f44dbf4c6535264a472322322705133b11/packages/protocol/contracts/L1/provers/GuardianProver.sol#L46 
https://github.com/code-423n4/2024-03-taiko/blob/f58384f44dbf4c6535264a472322322705133b11/packages/protocol/contracts/L1/libs/LibProving.sol#L192
The GuardianProver contract is a multisig that might contest any proof in some exceptional cases (bugs in the prover or verifier). To contest a proof, a predefined number of guardians should approve a hash of the message that includes _meta and _tran.
The issue arises from the fact that the approved message doesnt include the _proof. It means that the last approving guardian can provide any desired value in the _proof. The data from the _proof is used to determine whether it is necessary to return the liveness bond to the assigned prover or not:
As a result, the last guardian can solely decide whether to return the liveness bond to the assigned prover or not.
The decision to return the liveness bond depends solely on the last guardian.
Consider including the _proof in the approved message.
dantaik (Taiko) confirmed and commented:
