I got rid of my last microwave oven about 8 years ago.
I once had one which
did set afire.
I was using it when I saw a flash shoot from right to left inside the oven area, and found a large hole where the interior casing was blackened and melted on the interior right side.
I had been using it properly and the damage was NOT due to the food igniting, but the part which emits the wave had burnt through the interior casing.
I dont recommend their use anyway because they do alter the nutritional value of the food (for the worse)
Microwave gag order removed