Do some lenses focus in lower light?
I have the d600 with 50mm f1.8G. I have a feeling that sometimes it isn't locking focus when it should. This happens in low-light situations only. I was out with another photog who felt her tamron shot zoom (f2.8) was locking focus much more reliably than her copy of the 50mm f1.8G also. this is only anecdotal. i've tried to find some articles discussing this but come up with none. i can imagine that some lenses are more contrasty and that might help. But that's just my imagination.
I only use af-s center-point.