^this. A heater is the most likely thing to crash your tank, so you should always include special precautions to prevent that. I always do the following.
1. Use 2 or more heaters to get the required wattage rather than 1 large one. Don't overdo the wattage. Use a calculator like this and try to stay just above the wattage to maintain temp.
http://www.kernsanalysis.com/HeaterCalculator.cgi
2. Buy heaters with reliable thermostats and not the dial built into the thermostat body type.
3. Use a controller (apex, rk, ranco, etc) for primary heater control and set the thermostat a degree or two above normal tank temp as a backup. If the heater controller fails you will know because the temp will be higher than normal, but still a safe level that won't kill anything.
4. (Optional and only with aquarium controllers) Set the state so that in the event the aquarium controller loses communication with the power bars only half of the heaters can run off the native thermostat. Do the same in the event the temp probe falls out of the water or gets unplugged (If Temp < 70.0 Then OFF for apex).