Microwave ovens run quite hot anyway but they have a cooling fan which draws cool air usually at the back of the oven. Microwave oven cool air vent
However, if this air vent becomes blocked, which it does over a period of time. it draws in fluff which builds up like this :
Eventually the oven will not be able to draw enough cool air through the vent and the workings inside will get progressively warmer until one or more of the microwave components overheats and fails. microwave oven magnetron distorted through overheating[/caption]
This particular blockage resulted in the failure of the magnetron, the device that produces the microwaves. The microwave overheated and cut out. It was repaired and it wasn't too costly for the customer but could have been prevented completely.
Look at the back of your microwave and see where the air is drawn in and just check the condition and clean it off with a stiff brush - but not while the oven is running or the fluff will be drawn inside the oven. if it is bad then you may need to remember to check more often to save the cost of a repair.