We're Jammin'

"Nikon has received reports of a small number of D4/D800 users who are experiencing 'locking up' issues, where the camera becomes unresponsive until the battery is removed and reinserted. Nikon is investigating a permanent solution to this issue and will update users accordingly. To immediately resolve this issue, users can turn off both the 'Highlights' and 'RGB Histogram' display options in the 'Playback Display Options' sub-menu found in the 'Playback' menu" Source: Nikon USA

Flashback to the late 80s and the FM2's MD12. A photocall (remember those?) never passed without at least one photographer exclaiming loudly before wrenching the motordrive off the base of the camera to push down the protruding pin which had jammed in the 'up' position and rendered the camera inoperable.

A "small number" can be interpreted as all D4s and D800s shipped to date and given that the majority of those units will be in the hands of professional photographers who received their cameras first through the NPS priority scheme, it's a serious fault. Professional photographers are most likely to be working with both the highlights and RGB histogram displays turned on and pulling the battery to free the camera will of course mean the loss of any images in the buffer.

This speaks to a lack of adequate beta testing and in particular, testing by professionals in the field prior to release where this would have been quickly unearthed.