Today, I am sadly reminded of what to do when you have a jar break in the canner. You can reduce your aggravation ten times if you act calmly and swiftly.
First, how do you know you broke a jar? TWO ways: you hear a big pop and go look in the canner and see jar contents floating around, or you see a jar that was formerly underwater floating merrily on top.
This happened to me today, and the fruit was still in it - the liquid had simply fallen out, creating air.
Next action? Stop for a second and THINK: How can I make this situation better without burning myself, ruining the other jars in the batch, or otherwise causing harm. If the jar exploded, the best course of action is do nothing until the batch is done. If, like my experience today, you have a jar that lost its bottom only (likely because it wore out), you can grab it with your tongs and slowly remove it to your sink. The stuff will likely fall out into your canning water and this is OK.
With the jar safely outta there, decide if you can skim out the jar contents from your canning bath water. Tomato Sauce or salsa will not skim out very well, but my bunch of peach slices today came out easily with a slotted spoon.
Why skim stuff out if you can? Because if you can avoid boiling your jars for half an hour in ever-thickening peach water, for example, you will have a much easier time later cleaning the jars that did not break.
And clean them you must. The final part of taking care of a jar break is cleanup. After your jars have cooled for 24 hours, clean them with soapy water and/or vinegar if it is needed. Carefully clean your canner after the batch is done in a way that keeps glass shards out of your sink, hands and off the floor (I usually pour the water through a fine sieve outside). And go on canning.
I hope it goes without saying: Don't eat the food that came out of the broken jar. What might look like a simple, clean jar break could leave bits of broken glass in the food. You don't want those rattling around your intestines later or cutting your mouth up.
In related news: Golden Harvest: Mason Jars. You failed me! You are only three years old and you lose your bottom? Disappointing since I have 50 year old jars that are still going strong.