Coming from a country background with a Lot of home canning in glass jars, there are stories of finding stashes of canned (jars, not cans) goods fifteen and twenty years old that have been perfectly preserved. Not recommended, of course, but I never think anything about opening a can of green beens three years old that was home processed. It’s a simple enough procedure and is a satisfying hobby!
My mom and I both can jam the same way: Cook it, jar it, tip the jars over for a few minutes, tip them back up, and voila! We don’t even put them in a water bath canner. I’ve never had a jar of jam spoil. Everything else gets the proper treatment though. The only home canned food I’ve had spoil is jalapeño jelly (which I water bathed), and I think it’s from the liquid pectin. I’m not a fan of liquid pectin.
When Mom and Dad moved back to the family homestead in 1995, they found several cans of stuff Grandma had “put up” still in the basement. Grandma died in 1948. Of course, Mom threw out everything, but emptied the jars so they could be sterilized and reused. She said that the contents still looked and smelled fine.