tomato holes

When I was ready to can some tomatoes this year, I checked out last year's remaining jars and found one that was caked with mould and just half full.  Of course, a jar that isn't full will not form a good airless seal, and will go mouldy, but this mould was on the outside, not the inside. 

When I washed off the mould, I found what looked like a bullet hole in the bottom of the jar.

In all my years of canning, I've never seen such a thing.  Maybe the stress of the hot water bath opened a hairline crack that was already there.  And then maybe it banged against another jar.  It took a whole year for just half the contents to seep out.