yep, I used to boil my jars for sourdough starter to clean and that happened. Found this:
Question:
The bottom of my mason jars fell off while canning salmon. What caused this to happen?
Answer:
The problem is thermal shock breakage, characterized by a crack running around the base or lower part of the jar and sometimes extending up the side.
Possible causes:
1. Filling cold jars with boiling food and liquid.
2. Filling jars with cold food, water or syrup.
3. Not using a rack in the boiling water or pressure canner.
4. Setting hot jars to cool on a cold surface.
5. Setting hot jars to cool in a cold draft.
6. Splashing hot jars with cold water.
7. Glass damaged by cleaning jars with soap impregnated steel wool.
8. Glass damaged by using a metal utensil to remove air bubbles.
Possible Solutions:
1. Jars must be kept hot until ready to fill.
2. Food must be kept at room temperature and liquid must be boiling.
3. Jars must be kept at least 1/2-inch above the bottom of the canner during processing. 4. Set jars 1-inch apart on a rack or towel to cool.
5. Cool jars at room temperature, away from drafts.
6. Cool jars where they cannot be accidentally splashed with water.
7. Use nonabrasive sponges and cleaners when washing jars.
8. Use a nonmetallic utensil to remove air bubbles.
