Some Key Points
You should always stay close to your canner during the beginning of your processing. This will allow you to hear any cracks so you can resolve the issue as soon as possible. A broken jar in a pressure canner can become a projectile that can even break the other jars in the canner so it is best to be prepared.
If a jar breaks while you are canning take a deep breath and plan out how you want to clean the mess up. Make room for where you want to move your canner and find the right tools that can help take out the broken glass in the canner. The longer the broken glass stays in the pressure canner the higher the risk of more broken jars and the more glass you will need to clean up.
However, if you are doing a boiling water bath and it is almost finished, then you can wait till completion before cleaning up the broken glass.
If a situation occurs where you need to take out jars before the processing is finished, remember to take care of them. A wooden counter or a counter with a towel on it are the best options to properly place the hot jars. If you put a hot jar on a cold surface the temperature change can cause a crack in the glass.
Always properly inspect your jars before using them for a recipe or project.