The Nikon D800/D800E and D4 had the unfortunate tendency to ignore focus priority under certain circumstances. I noticed this when writing my D800/D800E book, but was told that the bug would be fixed by the time the book was published.
So, I went ahead and described the procedure for using trap focus on pages 173-175. Unfortunately, Nikon decided to call this bug a feature, instead, but finally acknowledged the error with the latest round of firmware updates. I can now say that trap focus does work as I described, and that I will never assume that Nikon is going to fix something without allowing them, say, two years to get around to it. The bug is fixed permanently in the Nikon D810 and D4s, too. Sports photographers rejoice!