Cables "generally" won't do that unless there is a burr at the end of the cable housing. In my years of working in bike shops years ago and the hundreds of cables I replaced I don't recall more than a handful were defective out of the bag; however, I often encountered cable housing with a burr on the end from a previous person not cutting it correctly. A burr will do that to a cable in short order depending on how sharp it is and if it's burred to the inside (cable runs directly against the burr).

I agree with Velocivixen, a good shop should have caught that. You wouldn't have stripped it by loosening it. It may have been over-tightened in the shop and only had a couple of threads left and your tightening it back may have stripped it the rest of the way. Those bolts are easy to strip if you don't work with them all of the time and get a "feel" for them. Luckily replacements are either cheap or free at a shop.