Most microwave door switches can be tested for continuity to see if the switch has failed. This can "fool" you sometimes. This switch looks fine, tested fine with an ohm meter and even made the "click" noise ok when we pressed on the actuator button. 

BUT,

Once we opened up the switch, it became clear this switch's contacts where being overheated and failing = new switch time.

This switch "looked" ok, "checked" ok with an ohm meter...but as you can see...it is not ok.