To get the car all road legal it needs to be crash tested - that is usually what the pre-volume builds are used for. If they fail (and they sometimes do, because you can't do a finite element analysis on an assembly as complex as a car in the variety of scenarios that you would crash test it) then you need to reengineer the car. This usually means panel gauge increases, sometimes it means additional reinforcement parts (boron steel inserts and brackets a lot of the time). If that happens the production could be delayed by months as they'd need to find a way to feed those parts to the line and modify any tooling that might be effected.