Today I picked up this 1915 penny, because it clearly shows a common fault. The "halo" around Britannia is in fact the image of the king's head showing through from the other side. The photo does not show it, but part of Britannia is recessed around the left side of her waist. I read that this happened because the design of the head consumed so much metal from the flan it affected the strike.
Anyone got any photos of other interesting faults?