Solve the Microwave Quandary: 4 Roommates and Boiling Oil

[NewGuy1]

Hello
It would be awesome if anyone can help me with this.
The center of the top wall of my microwave oven seems to have melted and the coating has formed boils about 1cm in radius. My roommate thinks its because of the evaporation of oil in the food that someone microwaved for too long, sticking to the top and melting the plastic coating. He says the boiling point of oil is about the same as the melting point of the plastic, and therefore that was what happened. This doesn't seem plausible. I think its because of someone microwaving something pressurized (there are four of us using the oven and no one is confessing to misuse.) What do you think happened?
Background: We mostly use the oven to microwave vegetables or frozen food. Nothing more than 5-10 Mins tops.

 

[Pythagorean]

Smoke point of oil is around 150-250 C, well before the boiling point. Plastic can be a lot of different things depending on the kind it is... I'm sure they use a high-tolerance plastic for microwaves, probably higher than 250 C.

On the other hand, localized thermal runaway can occur in microwaves. But I don't know what real world conditions cause it to occur (I'm not an engineer). They're not like conventional ovens where they monitor maintain a constant temperature though; microwave ovens just keep dumping energy into the box the whole time they are running.

 

[etudiant]

More likely one of the users put in some dish or package that was not appropriate for microwave use. If that caused a hot spot from reflected/focused microwaves, you could get some internal damage to the oven.
Do note that the wrong stuff in the microwave can produce flames, which would also do the kind of damage you describe.
The consolation is that this should not affect the functioning of the unit, even if it looks bad.