I think possibly that my rarest coin is an 1887 6d identified as one type among very many patterns that exist for that year (and none have more than subtle differences):
As you can see, the first 8 is struck over another 8; however the feature that first drew my attention to it (and why I bought it) is the big gap between the second 8 and the 7. The inner circles of the 8 point directly to a space, but on the normal example they are to the left of a bead. The tail of the 7 points exactly to a bead, rather than to the left of it.