The Nikon D3 “virtual horizon” feature apparently has limited accuracy—pay close attention and don’t throw away that bubble level just yet!
Rotating the image below by 1° counter-clockwise makes it perfectly level. While 1° doesn’t sound like much, it is obviously “way off” to the human eye. To be fair, in other shots it was only off by 0.1° and 0.4°. The 1° figure might be a worst case; Nikon should tighten up the tolerances for what is considered level.
Nikon D3 leveled using “Virtual horizon” feature
Nikon D3 leveled using photographer’s eyeballs