Many years ago, I actually had a microwave
intermittently turn on by
itself... We (usually) took to leaving a jug of water in it while
quickly sourcing a replacement.
This may not be the best idea, unless your water is "dirty" (in the
sense of having something suspended that forms nucleation sites for steam bubbles). Our
water is "clean", and my wife once decided to try to heat some up in our
microwave. It superheated by a noticeable amount, then finally boiled off
catastrophically. Big cloud of steam, microwave door blown open, loud noise, 100 C water
splashing around the kitchen. There was some damage apparent later in the switch that
should normally turn the thing off when the door is open, which I eventually had to
replace, but otherwise the microwave itself was OK.
I was always under the impression that that sort of story was an urban
legend. The physics isn't out of the question, I just can't imagine
any container in a household that wouldn't provide a sufficiently
rough surface to nucleate. Surprised to hear it's a thing!