What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
The 1872 Seated Liberty Dime had a major doubled die reverse (DDR) error and was first discovered by Dave Thomas in 1999. The second hubbing was struck and rotated 175 degrees from the first strike. One way to identify this coin is to note a doubling of the words "ONE DIME" on the reverse of this coin. In particular: It will look like a small line striking through the middle of the "O" in ONE and the "D" in DIME. A small line inside the upper part of the "E" in ONE may also be seen as depicted in the example image of the coin above. More specific details for identifying this error and many others can be found on Gerry Fortin's site that specializes in Liberty Seated Dime Varieties.
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD