On modern cans the lid will be depressed inward from air pressure. If this lid is popped upwards (while still sealed) this shows that biological activity has ch

On modern cans the lid will be depressed inward from air pressure. If this lid is popped upwards (while still sealed) this shows that biological activity has change the air pressure within the jar... so it wasn't properly sterilized/seal on production. I'm assuming these are Mason Jars. Which have been in use since 1858 (the patent has long expired so they're almost universal). When dealing with something like pickles the seal isn't as important as the pickle juice itself is incredibly hostile to bacterial life. The seal is more important to prevent evaporation.