Found some good reading:
One of the most common examples of Anti-logic we see at Lensrentals is the statement The lens is soft/frontfocuses/backfocuses. Now don’t get me wrong, there are bad copies of lenses out there, as best we can tell ranging from 3% to 7% of lenses. And we know, despite our checkout procedures, that 1 of 400 lenses or so will be damaged in shipping and arrive not functioning. Sometimes there’s actual damage or misalignment of an element in the lens, although the vast majority of the time that’s not the case. Usually the subject of the photograph is soft because the lens is not focusing precisely.
Three to 4 times a week we have the following conversation:
“The lens you sent me frontfocuses, its not good.”
“OK, we’ll overnight you a replacement.”
Then the first lens comes back and its perfectly fine when we check it out. But the customer is very happy with the replacement lens, it worked great even thought the first one didn’t. So what has happened? Its rather simple, actually, and like most examples of Anti-logic it stems from a wrong assumption: the customer knows his/her camera is ‘fine’ because it works with fine with their other lenses – none of them front focus or back focus.
Read more here: *Link Removed*