The Good-copy/Bad-copy syndrome

Is it really true that there is so much quality variation in Canon (Zeiss and others) lenses that you may end up buying a "good" or a "bad" copy?

Surely, manufacturers must have some sort of quality control procedure for their products, particularly their prestige products ("L" lenses, Zeiss lenses).

And how do you know if you have a good or a bad copy: shoot a brick wall? Come on...