I noticed this a few days ago with my 1 month old D300. It will occasionally miss focus. I shot a wedding yesterday and it became clear it was a problem and not a user error. On more testing today, none of my lenses are mis-focusing a test chart. However, if I add another object near the focus point I'm using, even on single point AF and single servo, the AF picks it up, causing what I had selected for focus to be out of focus. I noticed this in the church yesterday, when photographing the bride and groom the D300 would pick up focus on guests behind. Or it would pick up focus of the altar, etc. Has anybody had any similar problems? Or have any idea how long a problem like this would take to fix as I would ideally need it back for 6th September for another wedding.