just push the bkt button left side of the body and see if it is set to some bkt mode - it will do this stuff even in manual by changing the metering, and it can be +/- 3 stops so quite wide. Good news is, if you shoot RAW, the files a D600 producers can easily be moved back to usable exposure in most cases.
Just tried the following settings on the D3S.
ISO sensitivity - 200
ISO sensitivity auto control - ON
Maximum sensitivity - Hi3
Minimum shutter speed - 1/4000
After putting the camera in "M", setting the shutter speed to 1/500, aperture to F/16, no matter the scene the shutter speed did not move from 1/500. The ISO jumped easily to Hi3, but even if what the camera saw as "proper" exposure could not be achieved, it would notify me via the meter, but it would not force the shutter speed any slower than 1/500 even at an underxposed Hi3. It operated exactly as expected.
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