I was thinking more like stray capacitance issues, or I2C resisters of wrong value (3K3)- something of that nature; maybe battery back-up voltage (3.32V) issue. Wiring the battery could hardly be easier and I've looked, until my eyes crossed, at the holder to be sure there was nothing wrong with it. I think I'll try a different battery, but I've already tried a different 1307 and a different 12.5 pF crystal, verified wiring- exact same result. Circuit is rock solid, just goes "404" on me when the battery is inserted.