LiquidationEngine - A bug in the LiquidationEngine could lead debt or collateral being assigned to addresses from where they cannot be recovered. Compared to MCD, the LiquidationEngine can call external contracts that are meant to save SAFEs by adding more collateral in the system. These "insurance" contracts, if coded incorrectly, can change system state without actually adding any collateral and thus block the engine from starting new auctions. The liquidateSAFE(bytes32 collateralType, address cdp) function also uses mutexes to prevent re-entrancy. If a mutex is not unassigned at the end of the call, it can prevent the LiquidationEngine from liquidating a specific SAFE in the future.