metal in the microwave!

i recently asked Caroline to fix me a cup of coffee, which means, pour it and add creamer, but since the coffee was cold, I also asked her to stick it in the microwave as well. when she handed it to me, she had left the spoon in it. The spoon was HOT! I said caroline, did you leave the spoon in the mug when you microwaved it? her eyes got all big and she said yeah, well, no harm was done, i didn't hear anything blow up, i did dump the coffee, but the microwave still works. yikes!