I have never once (unintentionally) superheated water in a microwave, and I've been using them since about 1980 (and God knows we were idiots with them back then).

It just doesn't happen - there are too many imperfections in our containers, and too many minerals for it to happen much.

I've experimented many times, and the reality is you have to work at superheating water in a microwave.

For me, it's taken things like a brand new Pyrex measuring cup (glass), and filtered water. I can do it with other stuff, but I've had to boil/cool it multiple times, something that isn't really going to happen.