I came across two of the worst examples of this a while ago when I was still dabbling in repairing cameras and lenses for my own use. I was asked to look at an aftermarket lens made by a well-known manufacturer. The lens did not focus to infinity, even though there were no outward signs of abuse or that otherwise anything was wrong. I had no choice than to partially disassemble the lens. Since not all SLR and DSLR cameras have the same lens to film plane distance, this has to be taken into consideration when making an aftermarket lens. Rather than making the lens specifically with the appropriate measurements, a general lens to film plane distance is used. Then the rear of the lens has a threaded section which allows the lens to film plane distance to be changed to accommodate different cameras. After adjusting the lens via this threaded section to assure proper infinity focus, these threads then have to be locked in place. This is usually done with some set screws. However, this manufacturer apparently decided that additional costs could be saved by eliminating the set screws and to use a piece of tape instead. I am not exaggerating; they used a thick piece of tape around the entire threaded infinity adjustment. The problem with the lens was that the tape had partially lost contact which had allowed the adjustment to change.