Put metal inside the microwave oven usually kill your oven, but why only “usually”?
Microwave oven use magnetron to direct microwave to metal box (this mean metal can also appear inside a microwave oven but just some special things). These microwave will bounce around inside the metal box until they are absorb by other molecules and the result is those molecules heated.
In the other hand, metal (depend on type, shape, etc.) will reflect the wave. This also have potential to create electric arc or electrical spark, those things will hit the metal box and the magnetron also then kill the microwave oven.
So in general, don’t put metal inside the microwave.