Squirrel? Woodpecker? Either way, that's a lot of acorns. https://boingboing.net/2015/11/19/an-animal-filled-a-microwave-a.html
The poor squirrel was just confused. It thought that "chestnuts roasting on an open fire" might also work if they stuck it in the microwave. Sadly, wrong type of microwave. (Plus, there was no "start" button.)