Can a cup of coffee explode if heated in microwave? Hello this is a serious question, and I dont know if I am really in the right part of stack exchange:

A friend of mine was trying to make a coffee in her lab, she used the microwave to heat the water (I hope you enthusiast do not crucify us for this bewildering savagery). She set the timer to 2.5 min, but then had to run some work. After returning the water had already cooled beyond usability, thus she restarted the heating process for approx. 30s. Before reheating she realized that there was a lot of steam in the microwave cavity. 
After the reheating she takes out the cup, and after having taken it out, the water in the cup explodes in her face. 

Some word on the used hardware:

The microwave is an old model from Carrefour home
The cup is an ordinary ceramic cup with shape of a truncated cone. 

This is not a joke, she did really burn herself and was lucky that she chose her glasses instead of her contacts. However more interesting is, how this explosion came about. There must have been some pressure build up inside the cup but how would such an inhomogeneity in such a small volume be explainable?
As others have stated, superheating can happen in a microwave, especially when using a smooth vessel, e.g. a new, unscratched cup. It is more likely in pure water, but not unheard of from other liquids.

But you need a hack:
The key to prevent superheating and a subsequent flash of steam when the cup is moved or so is to supply nucleation sites, or, simply put, some rough surfaces or gas bubbles where the water can turn to vapour.


Many recommend using a teaspoon in your cup, but at least in some microwave models that can lead to "fireworks" of electric sparks. So questionable at best.
If you can get your hand at some lab equipment, get one of those glass sticks with a hollow lower end or a little porous ceramic stone. 
But probably the simplest way is floating a toothpick in the mug.
Or use an old, scratched mug, provided it is microwave-safe.