I do a lot of canning-started a few years ago but got a bit of a bug; I would classify myself somewhere between expert and novice, at least within my chosen domain of sweet jams/jellies/preserves. Today I was quietly reading and heard the telltale canning ping from an adjacent room-- sure enough a jar of quince marmalade I had processed about two days ago lost its seal-- the center of the lid had popped up. I decided to take the opportunity to check through my whole stash [about 500 jars- yes I've gone a little crazy] and found three other jars whose lids had popped up. I had a similar experience a few months ago. I always check the seal on my lids at least twice before storage- once after they come out of the water bath and once as i check/label/store them, usually 24-48 hours later. I would categorize myself as high but not super-high on the scale of canning 'hygiene' -- I always use new lids, heated in boiling water before applied, keep the jars in a low oven before filling, etc.. I have a rough idea of appropriate head-space, but i don't get out the ruler for each jar. I have two questions-- anyone know why this happens/how to prevent it, and also if you really need to throw such jars away? I always throw them away though it seems wasteful. In spite of dire warning that tell us to treat unsealed jars like nuclear waste, I have never seen any bulging lids, bubbling or leaking contents, discoloration, mold or other obvious signs of rancidity. In fact, when I open the jar to throw its contents away, the lid is usually pretty tightly attached and has to be pried off even though the center popped up. The contents have never looked any different than their sealed compatriots as i guiltily glop them into the disposal. From my various perusals, the late-stage lid failure is likely due to a 'poor original seal,' but i am pretty careful about my canning procedure, processing times, etc.. I don't, unfortunately, have a root cellar or similarly dark, stable environment for storing my jars, but i do put them in a closet or room with the blinds drawn in the downstairs of my house where the temperature fluctuates least. I guess the only thing possible culprit I have noticed is with pretty much every case of new jars i get the lids are stuck tight to the jars, have formed some kind of seal, and have a strong jar rim indentation. Is there any chance these new lids have effectively become 'used' via the thermal changes the empty jars have undergone through their brief lifetime, and are not creating a good seal? Any help would be appreciated-- thanks in advance (and sorry for the long rant!)