I had this same issue with my counter depth GE microwave. Fan and Light would both turn on any time the door was opened. It was the top (and primary) switch that was bad. It read closed circuit with door open or closed Replaced it and up and running just fine.
I took the bad switch apart and saw what happened. There was carbon build up on the pads, and when the microwave was turned on for cooking something big (like being on for 7 minutes) the resistance in the switch due to the carbon would create so much heat, it melted the switch so that the contacts were always closed. Replaced with a better quality switch so hope this does not happen again. Thanks Adam for the tips!!