http://www.fsis.usda.gov/FactSheets/Microwave_Ovens_and_Food_Safety/index.asp#16

Arcing (pronounced "AR-king") is sparks inside the microwave oven caused when microwaves react to gold paint on dishes, twist ties and other metallic materials. Some foods such as raw carrots and hot dogs can cause arcing while being microwaved. In hot dogs, this can be due to the uneven mixing of salts and additives. In carrots, it can be due to the minerals in the soil in which they were grown. Whatever the cause, turn off the oven immediately to end the sparks. Prolonged arcing can damage the oven and/or the utensil. If caught at once, arcing should not damage the oven. Remove the offending utensil or food from the oven and either substitute a microwave-safe utensil or cook the food by other methods.