I don't have a good guess as to what might have caused this. The microwave has a removable metal rack held by 4 clips, 2 on each interior side wall. During use this evening, one of the clips caught on fire. My wife was running it at the time. She didn't say there were any sparks. She said she heated some food and the plastic on one clip had started to turn black. She took the food out and ran it again to have a look through the door, and it caught fire.
Any ideas what could cause that??