Following on from my last post about melting chocolate, I thought I should add a post about defrosting food items in the microwave because frozen foods can have the same effect for the same reason. If you take something frozen from the freezer and put it straight into the microwave to defrost for a few seconds, until the surface starts to melt there will be no moisture present to absorb the microwaves and the oven will arc, the same as if it were run empty.
To overcome this, just let the frozen food stand for a short while. A minute or so will allow some moisture to form on the surface. When you do start the defrost cycle, there should be enough moisture to absorb the microwaves and prevent arcing which will damage the oven.