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No need to return it; mine is exactly the same. If you look closely, you will also see that it's more of a guaze lying on top, as opposed to a crack in a sensor. I would assume this is part of it's miraculous functioning - I have yet to find something it doesn't track on. You've made a fine purchase.
 
yeah its just the Darkfield technology design. I've also seen it on the Anywhere MX mouse.
 
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