I was getting ready to cast some bullets, and had about 4-5 lbs. of molten lead in my RCBS pot (it holds 20 lbs.) I normally batch-melt my scrap lead outside in a big old cast iron pot and make it into 1 lb. ingots, which are then later used in the RCBS pot.
This time, I had a small coffee can of various scrap lead pieces that included a couple one pound fishing weights. It had sat inside my shed all summer after having aquired it last spring. I picked up the 2 fishing weights and dropped them into the molten lead in the RCBS pot, noting the clunk as they immediately sank to the bottom. I no sooner turned away when BOOM! a loud explosion and I instinctively jumped away.
My immediate thought was a live round from my reloading area had somehow got into the pot (from the other end of the room!).
My next thought was to grab for the fire extinguisher on the bench, but a quick glance behind me saw no fire, only hot lead splattered everywhere!
The pot was on its side, with not so much as a drop of lead left in it. The stove hood ventilation fan above the pot was clattering in loud protest of its recent coating of lead, the linoleum floor was smoking as it heated under a layer of hot lead, and I discovered that I had lead adhering to my hat, shirt, denim pants, and boots.
I unplugged the pot and the fan, shaking like a leaf the whole time. It took me a few minutes to realize what had happened.
I couldn't believe my good fortune in that I was not hurt in the least. There was lead on my hat, on my shirt, my denim pants and my boots. I had on safety glasses and leather gloves, but they had no lead on them. Not a drop had touched my skin!
The two fishing weights were on the floor amid the mess, still round and unmelted. But one of them now had a peculiar and prominent pucker. Closer examination revealed the cause of it all.
The weight had a small casting flaw, so small I had not seen it. At some point, perhaps from the fishing weight being used for its intended purpose, the flaw had taken in water. Despite a long hot summer sitting in a coffee can, the water had not evaporated. A moments contact with the molten lead in my pot instantly turned that drop of water to powerful steam, and an explosion resulted.
They say you cannot possibly live long enough to make all the mistakes yourself, so you have to learn from the mistakes of others. Here's your lesson for today! Never add scrap from an unknown source to a molten pot of lead!

Shasta