Fallacious yet widespread and documented beliefs courtesy of Wikipedia.
Placing metal inside a microwave oven does not damage the oven's electronics. There are, however, other safety-related issues: electrical arcing may occur on pieces of metal not designed for use in a microwave oven, and metal objects may become hot enough to damage food, skin, or the interior of the microwave oven. Metallic objects that are designed for microwave use can be used in a microwave with no danger; examples include the metalized surfaces used in browning sleeves and pizza-cooking platforms.