It happens if the headspace was off - too little headspace left. Doesn't take much to make a difference. The smaller amount of air is evacuated from the jars sooner than from the jars with more air in them and a seal forms. Usually it holds fine but sometimes it is a weak seal because there was a bit of boil over on the rim. So I usually mark that jar, pop it in the fridge just in case, and use it first. Or you can always pop the lid, wipe the rim, new lid and reprocess.

Dave

Dave what's strange is they seemed to seal before I fully sumbmerged the jars in the canner. They were in the basket and the basket was hanging with the jars almost fully submerged for a few minutes while I readied three more jars. And I heard sealing sounds at that point.

Called fake seals. Also fairly common. During processing heat exposure can cause them to seal and pop, seal and pop over and over. So can cool air drafts. Some times they will do it just sitting on the counter when the contents are hot enough. We just don't always see or hear it.

But then I don't leave my basket hanging either. Each jar get jar lifted down into the basket submerged in the water as soon as it is filled. Either way as long as the seal holds they are fine. The ones I don't trust and put in the fridge just in case can go to the shelf after 48 hours if the seal is still good.

Dave

Ohhhhhh thanks so much for explaining! I'll lower them down right away from now on. The main thing to remember is that as long as they are securely sealed in the end, it doesn't matter if they were fake sealed before going into the BBW.