This report investigates fail-safety for unmanned vehicles with autonomous functions, through the lens of fault detection, diagnostics and handling. A vehicle's ability to perform during a mission is limited by its fault prevention, avoidance, safety tolerance and recovery capabilities. These faults can lead to accidents with consequences that needs to be handled in the design and use of the vehicle. A risk analysis is vital in finding critical faults and solutions often take the form of on-board diagnostic systems. The report also examplifies common hazards for systems in the domains ground, air and sea, above and below the surface. For high level functions, many similarities exist among the domains. Perception and control system faults are the most safety-critical and the causes are often hard to identify, especially if data-driven methods are involved. Aside from these faults, among the most common hazards are actuator, sensor and drive train failures, electrical failures and loss of communication. An important conclusion from the report is that endurance, cost and the complexity of assigned tasks increase the need for fault diagnostics.