[Silverback51]

If you look at the 009.jpg picture you can tell that this condition has existed for some time. The final failure happened when the dark gray area failed. The lighter gray area had been cracked for some time. It appears to me that it failed right at the transition to the threads. Is that correct?

And don't feel bad. The chances are that a casual inspection would not have indicated the condition so you did not do anything wrong

I would recommend that all ball joints be replaced. Could have been a bad batch. or an isolated condition.