QC is never perfect (like you said), so don't fret about it. Even top tier groups have slips in QC sometimes. If you check your work as you go along, it definitely helps minimize any errors, but it's entirely possible for something small to elude everyone's attention until release.
EDIT: Neg-repped for stating the obvious. Of course our plan is to "shoot for the top", but nothing will ever be perfect. That is a fundamental law of life.