>Was out on a shoot yesterday. >I notice the battery in my after-market MB-12 was getting low, >but thought the D800 should switch to the internal battery >which was fine.
You never mentioned the after-market grip before. If you were using the grip each time the problems occured previously, and if it's a non-Nikon grip, then it's most likely the cause of your grief. If that's the case, it would not be the first time that a third-party grip has caused headaches.