Cool Physics: Visualizing microwaves in a microwave oven (opens in new tab)

I think the carousel was the whole point: it gives us a three dimensional impression of the microwave intensity in the entire cube of the oven.

yea, that kind of made things a little more difficult, but I suppose it's realistic to actual conditions

It's simple to avoid if you're not intending to drink the glass of water, simply drop a toothpick in - break it if you want to be extra sure. The toothpick has a rough surface, if you break it then you're making an exceptionally rough nucleation point between the jarred fibers.

IIRC it's not so much a clean glass as a polished glass. If it's a regular cheap ass glass then it has enough imperfections that it's unlikely to ever happen with tap water. However polished glass (where the inside is actually sanded/polished smooth to remove imperfections) is the dangerous one, especially with distilled water (why anyone would drink distilled I don't know) or I suppose even filtered water would increase the risk exponentially compared to tap that can contain sediments from pipe scum.

Note that it's mostly distilled water you should worry about:

http://www.youtube.com/watch?v=1_OXM4mr_i0 (Mythbusters exploring it)