Nikon D4 Focus Issues

I was shooting a graduation ceremony last night and at the worst possible time, while the diploma was being awarded, the auto focus on the Nikon D4 went awry. There is no obvious reason that can explain why the subject would be out of focus while using Single-servo, Single-point AF, square on his face. This is the first time the D4 auto-focus system has failed me. All other photos, both before and after, were in focus. If anyone has any theories, please share them with me.