It depends on what you have the water in. A very smooth-sided container with very pure water has few nucleation sites and so the water is much more likely to become superheated, while a beaten up mug and standard tap water is less likely to superheat without boiling.
happens to my tea all the time--i don't know if it's actually superheated in the sense of being >212, but it sits perfectly still in the mug until i drop the bag in, at which point it goes nuts with the bubbles.
