Depends on the defect. I think easily more than half of all iphones have dust trapped inside. They're not manufactured in clean rooms, and there are just so many possible points of entry. Another common defect is the light leak. I personally didn't notice mine until I tried to look for it today. At first I thought it was a reflection off the chrome rim, but sure enough it was a light leak. If I didnt hear about light leaks here, I probably would have never discovered it on my own.
These are the types of defects that probably pass factory inspection since the vast majority of people will never notice it. (just like how an LCD panel can have X number of dead pixels before it is considered defective).