If the 2nd curtain hangs, and it is undamaged, then you get consistent overexposure.
If it jams, then you get a band of overexposure where it sticks.

If the artefact is curved like this, then your shutter has a piece of the blade damaged.
It could be the leading or trailing blade, and since it's over-exposed it is a piece missing from the shutter blade.


My shutter failure on the 1DSmkII was a wedge of over-exposure at the top of the image, which means the shutter was sticking at the bottom of the camera (the image is inverted on the sensor) and sticking more on one side than the other.

Because I bought from Canon's own ebay reconditioned outlet, it was fixed by Canon under the 1 year warranty.