Example of glaze crawling on the inside of a stoneware mug. Notice how thick it is. Thickly applied glazes have more ability to assert their shrinkage during drying and thus compromise their bond with the body below. The cracks that appear become bare patches after firing.
A ceramic glaze fault that occurs during firing of the ware, islands of glaze form as it crawls, leaving bare patches of body.
Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.