Actually, it was my first microwave fire. I put a small pizza box in the microwave thinking I'd just "warm it up" without dirtying a dish....
What I didn't realize is that the box was lined in some kind of metal/reflective highly flammable in the microwave material on the inside.
It only took about 10 seconds for it to ignite and luckily I was still standing right there.
Mmmmmm....the pizza is sure delish though.