I was in a hurry tonight and I wanted to cook some frozen coconut fried shrimp fast, so I decided to test one in the microwave (because it wasn’t recommended to cook them that way). I put one in the center of a corningware plate (which I use all the time in the microwave), and turned on the microwave. There was this huge arc: I don’t mean some sparkles, but an almost blinding light. Scared the shit out of me for sure. The microwave seems okay. I used it for something else later. But, I sure wouldn’t want this to happen again.
It’s not common, but microwaves do this sometimes. It happens when the current induced by the microwave field is forced to pass through a thin spot between two parts of a bit of food. While this does not damage the oven, it can be pretty alarming to have happen when you are not expecting it !
You can produce a similar spark whenever you like by placing a grape that has been cut almost in half (held together by the skin only) in your microwave with the cut surface facing up and turning on the power. The microwaves induce an electric field in first one half of the grape and then the other. When the resulting current tries to go through the thin area between the two halves of the grape the current gets high enough to vaporize the connection in an impressive spark.
For a movie of what happens to grapes, and other objects, in a Microwave see: Physics in a Microwave Oven