This is true, please heed my words. Microwaving liquids (especially in a coffee mug) can cause the liquid to explode upward as you remove it from the microwave.
I've only had it happen to me once. I was warming up some water because the coffee pot was in the dishwasher. I reached in, grabbed the mug, and then I noticed "hey, the water is shooting out the top of the mug". Ouch ouch ouch. Luckily the water wasnt too hot yet, and did not cause serious burns.
This mainly happens in mugs that are (1)new, and (2) in mugs that have very smooth interiors. The water builds up energy, and has no place to release it. The jarring movement of removing the mug causes it all to be released at once.