Making Solar Panels with a Microwave [video]

The magnetron burns out from a high standing wave ratio, which is the ratio of how much energy bounces back into it, to how much its outputting. If you aren't heating anything up, the microwaves bounce back and cause damage.

If you have something in the oven that absorbs the microwaves and gets hot, then you're fine. Liquid water does this, including the water in your food, but ceramics do it too. That is why when you cook frozen foods on a ceramic plate, the plate can heat up faster than your food. Once the food defrosts, it'll have liquid water and begin cooking at full speed. Sometimes the food only defrosts from the plate heating up, so you end up with food cooking from the bottom up, instead of the top down.