My Canon EOS 1Ds Mark II started adding a light line in the middle of all the photos. It almost looks like a light leak. It changes shape with different shutter speeds, f stops and ISO settings. I tried changing CF cards, batteries and lenses with no change. I put the camera in a changing bag and shot a photo and there was still no change, so it is not a light leak. I put the camera in sensor cleaning mode and inspected the sensor and I could not see anything wrong.
It turned out to be a defective shutter. Canon replaced it for $202.