Did I just almost start an oil fire

It's caused by the high amount of potassium in the banana. Microwaves react with metals, bouncing off and cause arcing. You can even create a cool light show by putting a raw peeled banana in the microwave. Don't worry, it won't explode, but it will make a mess, it's also harmless.

This can also happen in some frozen vegetables depending on the soil conditions they grew in. As noted in the linked article, other high concentrations of metals such as magnesium, iron, and zinc can be the responsible mineral.

Momentary brief sparking is harmless and won't harm your microwave, but the USDA recommends turning off your microwave if you see sparks nonetheless. If it does start a fire, unless it's a very small fire, do not open your microwave door. The safest thing is to turn off the microwave and let the fire burn itself out by consuming all the oxygen. Opening the door could create a dangerous backdraft induced fireball.

See Also:

• http://www.scribd.com/doc/1697458/USDA-Microwave-Ovens-and-Food-Safety
• http://www.inspection.gc.ca/english/fssa/concen/specif/veglegmicroe.shtml

In Spain, the traditional way is using something like this:

Or a pan with holes. The big day to do it is called Magosto and it's celebrated on 11/11, but it varies from town to town. It's one of the closest things to a barbecue, you first roast chorizos, then you roast chestnuts.

If you don't have access to those tools, your best option is to put them close to the fire. Make a small cut in almost each chestnut (to release moisture). The ones without the cut will pop and tell you when they start to be done. Start checking for doneness at that point.

Maybe you already knew about the cut, but I know some people don't roast at home because they don't know it can be done in a regular oven if the cuts are in place to avoid the chestnuts popping wildly.