Question about a burnt microwave?

Okay, so we bought a new microwave from the store and wemt on vacation. We left our elderly aunt to watch the dogs. When we came back the microwave was stained dark yellow and smelled like burnt plastic. We asked her how long she cooked her food (a pot pie) for, and she insists that is was only six minutes. So my question for the Yahoo Answers community is... how long do you think it would take to do this to a microwave? It is so burnt that the store wont even return it!