Let's face it heaters do and will fail. Doesn't matter what brand, preset vs. adjustable, or submersible vs. external vs. in line. These things are man made and will eventually fail. We have thermometers for when they fail to come on. Sometimes they fail in the "ON" position and could cook your fish especially if it happens at the wrong time such as when you are asleep, at work or school.
Does anyone know of a kill switch that could be placed on the socket that your heater is plugged in that is attached to a thermal probe that would break the circuit at the plug in the case of a failure? Like something you set at say 85* and if your temps hit it will trip and open the circuit to kill power to the fouling heater. Possibly a chime going off to let you know that your heater has failed in the ON' position.