Microwave Sparking Inside

The microwave threw sparks, there was a burning smell, and now you are wondering whether it is dangerous or just dirty. Tell it when it happened, what was inside and where the sparks came from, and it runs a fault tree across the usual culprits: burnt residue on the waveguide cover, a damaged cover, stray metal, a worn turntable ring or magnetron arcing.

My microwave sparked inside and there was a burning smell. This happened {WHEN_IT_HAPPENED} and I had {WHAT_WAS_INSIDE} in there at the time. The sparks came from {SPARK_LOCATION} as far as I could tell. Walk me through a fault tree to find the cause: burnt food residue on the waveguide cover, damaged or missing waveguide cover, metal in the oven, damaged turntable ring, and magnetron arcing. Tell me which causes are safe DIY fixes, which mean the appliance is written off, and how to tell the difference before I touch anything.