While I have been critical of Nikon recently for what I perceive to be QC issues - it is not like the competition is blameless.
Canon had to concede after the 5d mkiii release that some had a 'light leak' issue which caused erratic metering in darker conditions when using the top lcd's back lighting.
Not trying to defend Nikon as much as point out that it does happen now and then, and it is always frustrating, but it is as likely to happen buying a 5d as a D800 from what I have read. I would just pick the camera I want and find a good one. I am still on the hunt for a good D600 to pair with my D700.