Interesting example of a worst case scenario when the photographer forgets to cover the viewfinder during a long exposure, where sunlight is falling directly on the optical viewfinder. Light can track through the viewfinder and its prisms, back into the mirror box and affect the exposure.
Yes it was me and this happened during a 1 minute exposure using a Lee Big Stopper. Usually the effect is much more subtle, where the light leaking onto the sensor through the viewfinder path appears as a small, faint, pinkish tinge somewhere in the frame centre. The D800 has a great system to prevent this (providing one remembers to use it!). It has a small blind built into the viewfinder which covers it completely at the flick of a small switch. Much better than fiddling with small plastic clip-on blinds that come with many cameras, which will almost certainly get lost.
RAW straight out of camera. Looks a bit like a high resolution Holga with a bad light leak! I would like to claim it as artistic interpretation but, no, it was just a mistake.