1: Extreme temperature
While most hygrometers should work fine at "normal" indoor temperatures, it requires a hygrometer set up specifically to withstand temperatures below freezing or temperatures above the boiling point of water. For example, a thermo-hygrometer exposed to temperatures below -4°F or above 140°F will start returning false readings.
Severe high or low temperature hygrometers that are not specifically designed for extreme temperatures for extended periods of time will not only degrade the accuracy of woodworker readings, but will also cause lasting damage to the meter's electronics, permanently rendering them ineffective.
2: Exposure to moisture
Ironically, prolonged exposure to too much moisture can actually cause damage to the hygrometer. This damage can manifest itself in a number of ways, such as corrosion on the contact elements of the hygrometer (such as the pins of a pin meter) or failure of the internal circuitry of the meter itself.
Anyone who's put a phone in a swimming pool or other body of water can tell you that moisture and sensitive electronics don't get along well. However, you don't have to put your electronics in water to expose them to excess moisture. Leaving the meter in a wet environment, such as buried in a bag under wet work clothes, can cause damage over time, just like dropping it in water.
