Zener diodes are widely used to create voltage reference values in a circuit. In some situations, a very stable voltage reference is required. A common pitfall when designing such a circuit is to fail to get into account the temperature coefficient of the diode.
The temperature coefficient is a graph which indicates how the reverse-biased zener voltage changes when the temperature changes. The change ins usually in the scale of milivolts, some times its negligible but some times it can radically upset the circuit.
The following video is a very good tutorial from mjlorton in which he explains the temeprature coefficient graph, how you can read it, and how you can avoid a circuit failure. Also he runs an experiment to prove the point.