I had a car where the lug nuts had been overtightened that could have done this as well. I had a flat tire so was going to remove the wheel to have it fixed. One lug nut and the stud was just simply MISSING! The others snapped off easily upon me trying to remove them. Only ONE was still solid. The ones that snapped were hourglass shaped like they had been overtightened and stretched. Too tight is just as bad is too loose!
I am glad that tire went flat so I found this. Otherwise it might have let go at a bad time.