coffee stain in microwave
I had some fancy-schmancy freeze-dried decaf and it disappeared and hardened into the bottom of the jar before I even enjoyed a second cup. That whole jar of coffee concentrated down to 1/10th of it's original bulk and stuck to the glass. I wanted to recycle the jar so I filled it with hot water and stuck it in the microwave.
As I am wont to do, I forgot about it. The jar boiled over and shot brown gunk all over. I'm left with stains that are not brown, but they are not the same ivory color as the microwave interior surface, either.
I tried a little Bar Keepers Friend, and then a Magic Eraser to no effect. When those didn't work I thought maybe the stains were oily, like roasted beans can be, so I used some grease cutting pine cleaner. No luck.
There are hundreds of these little dots all over the place in there, each the size of a pea or smaller - the size that a chunk of projectile calcified instant freeze-dried coffee would make.