As most of you know you must use all cotton materials. They greatly reduce the risk of scortching and burning in your microwave. Like someone pointed out above you must be careful that it is free to move on the microwave turn table as well.
Another point that many people forget is that you must wash your potato bag! Food scraps from the potato can get left behind in the bag and after many uses can cause problems. A wash after every use is not needed but every once in a while will reduce the chance of an incident. Just some things to keep in mind.