Believe it or not, I overran the buffer on my D4 for the first time last weekend. I had a 32gb XQD and 32gb Lexar 1000X in the D4 and was shooting the Shaw AFB Air Expo's jet car / jet plane race with a wall of flames pyro.
It surprised me that I overran the buffer, as I had tried before and couldn't. But in this instance, shooting at 10fps, I overran it.
On my D3S I would rarely overrun the buffer, but it would happen at least once per air show. On my D300, I would constantly overrun the buffer.
BTW, I had some Air Force security officers looking around and looking at me with a wierd look when I was firing off the D4 in CH mode. They said the camera sounded a lot like a machine gun going off.