Tonight, as I was preparing my Too Tired to Cook Black Bean Dish, I discovered one of my home canned jars of tomatoes had unsealed. I was sad (because home canned tomatoes are far superior to commercially canned ones, and because I consider them precious) and surprised, because the jar had sealed when I originally canned it. However, it was a good reminder that occasionally jars loose their seal and that we should always check the seal before consuming any home canned (or commercially jarred) food. How do you check the seal? Sometimes you might see the jar lid bulging because of bacterial growth. Sometimes you might push down on the center of the lid and notice it moves. (The lid should be concave.) Other times, the only indication may be the lack of "whooshing" noise when you remove the lid. What causes lids to unseal? Moving jars, or banging other jars against them can sometimes break the seal. (This is why stacking jars one on top of the other isn't recommended.) Storing jars in hot or very cold locations may also make them unseal.
What should you do with jars with broken seals? The official answer to this question is the same as if we were talking about commercially canned or jarred food: Place the entire jar (including the still-filled glass container, lid, and screwband) into a heavy plastic bag, seal the bag shut, and dispose of the bag in the landfill. The idea here is to eliminate the chance of exposing other surfaces to dangerous bacteria that could make someone ill. The National Center for Home Food Preservation reminds:

"Contact with botulinum toxin can be fatal whether it is ingested or enters through the skin. Take care to avoid contact with suspect foods or liquids. Wear rubber or heavy plastic gloves when handling suspect foods or cleaning up contaminated work surfaces and equipment. A fresh solution of 1 part unscented liquid household chlorine bleach (5 to 6% sodium hypochlorite) to 5 parts clean water should be used to treat work surfaces, equipment, or other items, including can openers and clothing, that may have come in contact with suspect foods or liquids. Spray or wet contaminated surfaces with the bleach solution and let stand for 30 minutes. Wearing gloves, wipe up treated spills with paper towels being careful to minimize the spread of contamination. Dispose of these paper towels by placing them in a plastic bag before putting them in the trash. Next, apply the bleach solution to all surfaces and equipment again, and let stand for 30 minutes and rinse. As a last step, thoroughly wash all detoxified counters, containers, equipment, clothing, etc. Discard gloves when cleaning process is complete."

Unofficially - although I would never recommend anyone else do this - I dump the contents down the disposal, run the disposal for several minutes, adding some bleach, throw away the lid, and bleach the jar and screw band for later use.