The ceramic mugs by their nature tend to stay very hot and cool down slowly. As long as the mug is still very hot, it is possible for the color to migrate beyond where you want it to be. You can avoid this by dipping the mug into lukewarm water following sublimation. Just take off the transfer paper and then dip the mug in the water. After a short time you can take it out and dry it off with a paper towel. This will help the mug (and the ink) to cool down and stablize.
