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Hardware gremlins?
Flip top to bottom the images and then stack, then see if the bottom area is still smudged. This could be a hardware failure: do an extensive memory and hardware diagnostic. The personal computers today don't do error checking and correction like the cluster workstations do, so a failure goes undetected unless it causes a problem. Also make sure that one of your images isn't corrupted as that will spoil the stack also.
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