Nikon has confirmed to PDN that it is investigating a problem that can cause the D800 and D4 to lock-up while shooting. The company says that the issue – identified while PDN was reviewing the camera – can be avoided by disabling Highlights and RGB Histogram on the display. At present the lock-ups require the battery to be removed to restart the camera but PDN thinks it likely that Nikon will be able to come up with a permanent fix.

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News: Digital Photography Review (dpreview.com)
http://www.dpreview.com/news/2012/05/04/Nikon-tells-PDN-it-is-looking-at-D4-D800-lockup-bug