You may have an issue where two elements with a common background are meant to be seamlessly aligned with each other, but in the affected system and browser, the rendering is forcing a pixel or two of spacing between the objects, causing the background to bleed through. This would appear as though a line is rendered, when in reality it is just the background. That is why you won't find it in yourcode directly. If it is a table tag, make sure your cellspacing is explicitly set to 0.
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