What this coin looks like (obverse, reverse, mint mark location, special features, etc.): One major variety is the 1861/0 Seated Liberty Half Dime Overdate Error. This overdate features the 1861 date being struck over the 1860 date (1 over 0). This can be identified by noticing part of the underlying "0" appearing just to the left of the "1" in "61". A very small part of the curve of the "0" can be seen near the base of the "1" and just to the left or upper-left of the "1" as depicted by the arrows in the example image of the coin above.
Coin value chart: Typical coin prices, values and worth in USD based on grade/condition