When a shutter fails it typically does so by failing to close completely between images. The result is that the center of the frame is overexposed. Above is a dark image taken after a suspected shutter failure. Dark images are used to subtract out pixel noise. One collects charge in each pixel for some time (often a time equal to an exposure duration) while the shutter is closed. That is, the shutter is never told to open for a dark frame. The resulting signal can then be subtracted from the raw images since the counts are due to noise the camera is generating. I don't think the camera shutter was closed all the way when the above dark frame was shot, do you?