Next time you pop one of those convenient bags of microwave popcorn, put the bag on a plate first.  The bottom of the bag can become so hot that it can crack the glass tray inside the oven.  Another good suggestion for popcorn and other foods is to place the food off center on the turntable.  This enhances the stirring effect of the turning movement and also makes it less likely that the same spot will be used repeatedly for cooking.