I am shooting on the Nikon D800. I suspect it has a sensor defect because there is a very uniform artifact, in my images. It looks unlike a light leak as it's always in the same position and has the same character.
My theory is that in both circumstances, the image data was too little or took too long to build on the sensor due to which this is some sensor read-out pattern that dominates over actual data.
Image one: Sunset + 10 Stop ND + f9.5 + ISO 100- Very very underexposed.
Image two: Lens Cap on, f3.5, ISO 3200.