Lens Copies - is there such a thing as a "good" or "bad" copy of a lens?

I bought a Canon XT several years ago and although I noticed no focus problems with shorter lenses, I had focus problems with a 400mm f/5.6L lens. I thought that I had a bad copy of this lens.

It turned out that I had a good lens but a bad camera. The focus was off on the camera. I had never abused this relatively new camera it was a "bad copy"... I just had not noticed the focus being off a tad when using the shorter lenses.

The good copy - bad copy problem seems to be fairly prevalent in many types of Sigma lenses. I have a friend who went through three Sigma 30mm f/1.4 lenses until he found an acceptably sharp copy on the fourth try. These lenses were checked on his 50D and my 30D and 40D cameras which I now use. When he found a sharp copy, it was razor sharp. I also have read numerous posts on various forums stating the similiar problems with Sigma quality control.

However, I would expect that many bad copy problems can be traced to the indian rather than the arrow. This is especially true when shooting with longer lenses and using multiple focus points. Unless you are very attentive to where your camera is focusing, it can easily pick out a focus point which will throw your entire image askew.