What this coin looks like (obverse, reverse, mint mark location, special features, etc.):
The 1963 Silver Roosevelt Dime had a major error in one of its proof coin issues. Some coins feature a slight doubling of the words "UNITED STATES OF AMERICA" and the motto "E PLURIBUS UNUM" on the reverse of the coin. The most prominent and noticeable doubling occurs in the word "UNITED" as depicted by the arrows above. The doubling is far harder to spot in the "E PLURIBUS UNUM" motto. The thing to look for here is slightly wider letters and very slight doubling on a few of the letters - especially in the first two letters "E" and "P". These are also depicted by the arrows in the example image of the coin above. There are many varieties of this double die reverse (DDR) error and not all doubling will appear as strongly. The value goes up with stronger and more noticeable doubling.
Coin valuation chart: Typical coin prices, values and worth per grade or condition - in USD