Glaze defects are visible flaws that appear on a fired piece when a glaze does not fit its clay body correctly, or does not behave as expected during firing.
Crazing is a network of fine cracks from a glaze that is expanding more than its body. Shivering is the mirror-image defect: a glaze compressed too tightly, flaking off the body instead of cracking. Crawling is glaze pulling apart during firing, exposing bare clay underneath. Pinholing & blistering are craters and bubbles left by gas escaping through the glaze layer during firing.