I know this is tangential-- but does anyone know what the normal failure modes of microwaves are like? 20 years ago I microwaved some leftover macaroni and che

I know this is tangential-- but does anyone know what the normal failure modes of microwaves are like? 20 years ago I microwaved some leftover macaroni and cheese from just a few seconds-- and it came out absolutely charred. I was confused but then noticed the microwave making an awful sound--a low rumble--it wasn't supposed to be on at all. Then the sparks started, then the smoke. Unfortunately the plug for it was behind a refrigerator so I ran to the electrical panel to turn the house power off. I figure some mosfet or something that controlled power must have failed closed or something. It most likely would have burned the house down if I hadn't happened to be right there.