Reprinted from  http://www.fda.gov/cdrh/consumer/microwave.html

The FDA has received reports of serious skin burns or scalding injuries around people's hands and faces as a result of hot water erupting out of a cup after it had been over-heated in a microwave oven. Over-heating of water in a cup can result in superheated water (past its boiling temperature) without appearing to boil.

This type of phenomena occurs if water is heated in a clean cup. If foreign materials such as instant coffee or sugar are added before heating, the risk is greatly reduced. If superheating has occurred, a slight disturbance or movement such as picking up the cup, or pouring in a spoon full of instant coffee, may result in a violent eruption with the boiling water exploding out of the cup.

What Can Consumers Do to Avoid Super-Heated Water?

Users should follow the precautions and recommendations found in the microwave oven instruction manuals, specifically the heating time. Users should not use excessive amounts of time when heating water or liquids in the microwave oven. Determine the best time setting to heat the water just to the desired temperature and use that time setting regularly.