I assume this is a coarse thread shaft and a heavy hex nut?

Regardless, either the shaft is defective (too soft) or the nut you're using is severely mis-matched (stronger than) to the shaft. That should not have happened, especially not the way it did.

In a bolted connection, especially in a high tension connection like your ball joint, deformation must occur. This is so the tolerance between the internal and external threads is taken up. The nut's threads are put into compression, while the bolt's threads stretch in tension.

It's better for the nut to deform plastically and compensate for the tolerance in the thread lead. As it does, it distrubutes the load over many threads as it advances increasing tension. If the nut is too strong and does not deform, progressive shear can occur. This happens when the load is not distrubuted. The first thread takes all the load, fails, then same happens with the next thread and so on (Which is good in your case because it revealed a potentially greater problem with the ball joint).

The addition of the anti-seize would have an affect on the torque number, but I'm not smart enough to know how much. I can't imagine it would be anywhere near 10ft-lbs more on a 65ft-lb torque though. That's not the problem here.

What I wrote above is mostly paraphrased from a really good writeup on fastener design and applications at: http://www.rbwmfg.com/techInfo.html At that site click on the "Helpful Hints" link (it's a 2Mb .pdf file).

-Will
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850 / Mini