yeah mine did that...
i was worried that the splines on the stud were bad, but the end of the arm is fairly soft aluminum, from the looks of it, probably meant that way to A) conform to the splined shaft, and B) if one of them strips, it's likely to be the arm.... so after getting a new arm and tightening it down GOOD it hasnt been a problem since. -- though a bit of thread lock on the nut might not hurt.