If one liter
of water is superheated
by only 1°C, it is in an unstable
state, and can suddenly produce about 3 liters of steam.
It is surprisingly easy to do it with a microwave oven. The superheating only occurs in very smooth containers that don't allow bubbles to form. If bubbles could form, the water would boil, carrying the excess heat away*. The best way to prevent a cup of water from becoming superheated in a microwave is to place something in the cup like a teabag (no staple, metal in a microwave is very bad) or a microwave-safe spoon. This creates cavities and irregularities in the water where bubbles can form.
A quick and easy solution is to just drop in a toothpick before putting the cup in. It provides a surface for bubbles to form, is microwave safe, there is usually one around, and you can scoop out the toothpick with your spoon and feel it to determine if the water is hot enough without risking any injury.
*This is also related to weapons that fire caseless ammunition. As they have no brass to eject, they are in danger of overheating. The escaping hot brass turns out to be an useful addition to the cooling mechanism in an automatic weapon.