A few weeks ago I stayed at Down Hall Country House Hotel with some friends as part of a spa weekend. On the last day, we were taking some pictures, and my camera suddenly refused to continue and just said "Err" in the LCD display. Battery out, cards out, all back in - no change. Save settings, reboot - no change. After the panic had begun to subside, I eventually solved the problem (via Google) by removing the battery and cards and replacing the battery only and taking a couple of shots. This cleared the buffer by saving the images as demos and all was good again.
It turned out that the fault was with the battery pack I had attached (non Nikon). I use a Black Rapid strap, which i love, which attaches to the tripod mount under the camera, or in my case, the bottom of the battery pack. It would seem that the weight of the camera pulling on the pack may have affected the contacts. So I'm no longer using the battery pack and all is well again.
But it did raise another point. What if this happened on a shoot! I really need a backup. As I've only had full frame cameras, my lenses are all FX full frame lenses. I know they are compatible with APS-C cameras but a friend of mine (who bought my D700) has had some focusing issues mixing formats so full frame SLR is what I wanted.
So I got a D600 and what a camera it is. I'm not going to do a review here as the internet is awash with them but suffice to say this isn't going to be a backup but will sit along side of the D800e.