My SO says this is so, but I thought crazing was something that happened during manufacture, from cooling too quickly after firing, and that it just shows up eventually over time as stains eventually build up.
From Snopes: Yes.
Anecdote: I heated water in the micro in a coffee cup, put a tea bag in, and the water instantly boiled over. Onto my foot. I was unpleased.
um, but chique, that ain’t the question that de trop asked… 
my personal, non scientific experience has been that some ceramics craze (develop a pattern of really tiny hairline cracks in the glazing) over time from being heated and cooled. But I think that would happen from the microwave, the dishwasher, repeated filling with hot coffee, etc. I don’t think it’s exclusively a “microwave” phenomenon.
:: points at her blonde hair::
:: points at her Polack lineage::
::slips quietly away from this thread and blame her genetics::
:smack:
I have several different coffee mugs which have displayed increasing amounts of crazing on the interior as years go by. I’m always heating these up in the microwave, and I’ve always wondered if that makes a difference.
So the real question is: does the glaze get equally crazed from just pouring boiling water in the mug?
Speculation: if there are tiny cracks already in the glaze, and if water invades them, then the water in the cracks might heat up far more in a microwave oven than when hot water is simply poured into the mug. The microwave flux heats up the water that’s in the cracks. This wouldn’t happen if you simply poured in some boiling water; in that case the cold glaze would cool the water as it invaded the cracks.
Also there’s the phenomenon called “boundary layer” where a fluid develops an immobile coating upon a solid surface. If you just pour some water into a mug, the cold ceramic will cool the water it touches, and because of the boundary layer, this cool layer will not be stirred up by convection like the rest of the water in the mug would be. The cool layer would stay in place and, once cooled down, act as a cold insulating layer. Not so in a microwave oven, since the microwave flux tends to heat the outside of any chunk of water. The water which is directly in contact with the ceramic glaze would tend to get very hot while in the oven, but the same water if just poured into the mug would remain cool.
PS, the cure for “exploding coffee” is to stick a wooden stir-rod in your mug. That way the normal boiling will occur, and your coffee won’t superheat far about 100C and then explode when disturbed. Also, don’t boil coffee twice in a microwave oven, since the first time will remove all the seed bubbles, and if you try boiling it again, you’ll often get superheating and “BOOMF!” effects. See: http://amasci.com/weird/microexp.html#coffee