One typical failure is when the oscillators' stability is affected. This can cause it to stop, or change its period. The normal system hedges are to have an auxiliary oscillator using some cheap, robust scheme such as a ring of inverters or a resistor-capacitor one-shot timer. After a reset (perhaps caused by a watchdog timer), the system may default to these, only switching in the sensitive crystal oscillator once timing measurements have proven it to be stable. It is also common in high-reliability systems to measure the clock frequency by comparing it to an external standard, usually a communications clock, the power line, or a resistor-capacitor timer.