Definately over cooked! Perhaps a malfuntioning keypad but more likely operator error.
Panasonic micowaves of about 10 years ago had a delay cook feature. You could program them to come on at a later time. Some of these were put into RV's. While on the dealers lot, people touring the coach would sometimes randomly punch the keypad and inadvertantly set the micro to turn on later and for long cooking periods. Since it was "new", the cooking compartment contained the dish , owners manual and cardboard packaging. The metal staples on the manual would get hot and sometimes arc to the compartment side causing a fire. The fire was completely contained inside the micro but it sure ruined the "new coach smell".
BTW, Panasonic (Matsushita Electric) makes many models of micros for other brands including some GE models.
Watch out for the overcooked popcorn! It glows inside like a cigarette and may erupt into flames later. Don't throw it in the waste basket (but then you probably already knew that).