I think front(back)-focusing is purely a problem of the camera body, because the body controls and evaluates the focusing. The lens doesn't know whether it's focused or not, it is the body which does the quality control.
However, I see lots of complains on the lens having front(back)-focusing problem. Some even say that one lens has focusing problem but other lenses are correct on the same camera body. I don't understand this. It's like saying "This lens only shoots 5 megapixel image."
Of course, if the AF screw or in-lens motor fail, the lens cannot focus at all. But that's different from front(back)-focusing where the body confirms the focusing to be succeed.
Another possible related problem is the lag of the AF mechanism in the lens. When the body says "stop", the lens continues moving for a small amount of time. This can be fixed by half-pressing the shutter a few times. However, some people complains their lenses to be consistently front-focusing, and this method doesn't help at all.
Could someone enlighten me on this? What are the possible lens characteristics which cause front(back)-focusing?