In more critical situations you may need your Arduino board to have as close to 100% uptime as possible, for example with remote Ethernet applications or you just need the sketch to have the ability to physically reset your Arduino (reminds us of the old BASIC goto command...). If this is of interest, Arduino enthusiast Alessandro Rodella has created just the thing with their watchdog timer circuit.
A watchdog timer is a circuit that can reset the Arduino board after a set period of time if the sketch stops running, freezes or otherwise has an error. Using a simple astable timer built with a 555 timer, the circuit can reset the Arduino if the timing circuit isn't reset by the Arduino before a preset period of time. For successful operation you need to write the watchdog capability into your sketch, however once implemented if your sketch crashes the Arduino will simply reset and start again.