Por alguma razão eu li passado "forno de 9 anos de idade" ... bem, aqui vai:
Ou isto: (Pessoalmente, eu seria um pouco menos frouxo em dizer, eh ... tudo bem)
Sparking and scorch marks in microwave. What's going on here?
It's possible that either the glass or the [frozen food] allowed charge to build up to the point of arcing. It's also possible that there is something wrong with the microwave. Try putting a cup of water in the microwave and running it for about 30 seconds. If there is no arcing then the microwave itself is probably fine. If you continue to have arcing then replace the microwave.
-engineer_comp_geek, straightdope.com
As microondas não gostam de gelo. - thenakedscientists.com