Were vegetables being cooked in the microwave?
When cooking certain vegetables in a microwave, an arcing effect may occur in or on the food itself. The vegetables may display small burns.
Dense vegetables such as green beans, carrots and green peppers have a higher amount of minerals in them than other food items. These minerals include iron, magnesium and selenium.
The minerals act like tiny pieces of metal and create arcing effects in a microwave. The sparks result as the microwaves reflect or bounce off the metal. The arcing does not harm the food but it does prevent it from heating thoroughly.
Extensive arcing can damage the oven's magnetron tube.