I came across something today I had never seen before. After failing to get a 1/2"-13 bolt started in a nut that I knew was good (because another 1/2"-13 bolt had no trouble with it), eventually I realized the threads were rolled in circles, rather than in a helix. To make sure my eyes weren't deceiving me I spun it in the lathe. A good 1/2"-13 bolt held up against it meshes nicely, so the problem is only apparent when looked at closely. Maybe others have come across such defective bolts in the past, but in all the fasteners I've used in all the years I've been using them, this was a first for me.