Rare, but it happens. With so many manufacturers pumping out parts tolerance stacking, and burs are the most common problem. Reamers wear, shaving get left in bolts or barrel extensions, burs are left in gas ports ect, ect. This is why I like building my own. I get to inspect all of the parts, and make sure the parts are good as well as clean and oiled. The only AR I had that did not run the first time I took it out was a 9mm build. It took a little more fitting with that one. I just did a 6.8 SBR build with a friend and we found a few slight issues that we had to work through. Shot just fine the first time, and every time but if it were a factory build they would have not caught the problems just slapping it together.