It happened to me a few times. It turns out that I sometimes press the WB button with my nose when taking pictures. Then, I try to change the aperture or shutter speed with one of the dials, but instead I change the WB! Quite annoying if you don't notice. Not a big problem if you shoot RAW, as you can correct it in post processing.
Ummmm the D800 WB button is on top of the camera so unless you have a very BIG distorted nose this is not possible lol.did not think of that but it could well be the reason, because i do not remember changing the WB, after I checked it was on fluorescent.