The degree to which a planner succeeds and meets response deadlines depends on the correctness and completeness of its models which describe events and actions that change the world state. It is often unrealistic to expect perfect models, so a planner must be able to detect and respond to states it had not planned to handle. In this paper, we characterize different classes of these "unhandled" states and describe planning algorithms to build tests for, and later respond to them. We have implemented these unhandled state detection and response algorithms in the Cooperative Intelligent Real-time Control Architecture (CIRCA), which combines an planning system with a separate real-time system so that plans are built, scheduled, and then executed with real-time guarantees. Test results from the flight simulation domain show the new algorithm enables a fully-automated aircraft to react appropriately to certain classes of unhandled states, averting failure and giving the aircraft a new chance to achieve its goals. We are currently working to further improve CIRCA’s planning system, and to extend our detection and response mechanisms to other classes of unhandled states.