Well, the rust-ring of doom showed up on the bottom of the microwave, under the glass turntable recently and yesterday she decided to write a note to all users about cleaning their messes better. I told her the "mess" was rust, and she was causing it by boiling her pasta in water for 20 minutes in the microwave where as everyone else uses it for mostly 2-3, or at the most 10 minute cook jobs. She said I'm wrong because her bowl was large enough to prevent the water from boiling over the sides. I tried to tell her that boiling water in an enclosed area such as a microwave for 20 minutes created enough condensation to thoroughly flood the bottom of the microwave and even when the microwave oven is clean, sometimes it's hard to see the water, but eventually the little wheels that move the turntable start to remove the paint, causing rust and then bare metal which may then start to spark and catch on fire.
She doesn't believe a word I'm saying, so maybe I'm wrong. It's just my theory. Does anyone know the cause of the "mysterious rust-ring" on the bottom of our microwave?