from the following article :What Happens When You Put Metal In A Microwave?
it would appear that the damage would be to the microwave oven itself
my reckoning is that when they say it might be unsafe to use afterwards, that this might be due to electrical issues with the oven, not with the food become somehow inedible
of course, if the microwave works after a fashion but does not apply the expected wattage, you might have undercooked food, with all the issues following from that
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