What is unusual about this penny?

It's a Lincoln head cent, issued in 1909 on the centennial of Lincoln's birth, and the first in a long series of gazillions of similar pennies differing only in date and mint marks.
But there is something about this penny that no other penny on earth shares.  Answer below the fold... But there is something about this penny that no other penny on earth shares.  Answer below the fold...
It's not on earth.  It's on Mars.  Embedded in the framework of the Mars Curiosity rover, it has been used to calibrate the optical elements.
Image cropped from the original at Frankfurter Allgemeine, where there are additional pix of the aging of Curiosity.