The 1817/4 Overdate Half Dollar is the most famous and valuable of all the Capped Bust Half Dollars. Only ten examples are known of this variety, several of which have been discovered within the past decade. Most of the known examples are of the Overton 102a variety, which means they show a somewhat vertical die crack on the obverse. This crack, and the eventual removal or destruction of the die, is believed to be the cause of this variety's exceptional rarity.
The finest example is the PCGS AU53 from the Eliasberg Collection (and, coincidentally, the first example of the variety discovered by E.T. Wallis circa 1930). Wallis' asking price of $2,500 in 1934 pales in comparison to the $356,500 it realized in its last auction appearance in 2009!
For anyone who thinks that all of the great coins have been found, it is important to realize that four of the eleven known 1817/4 Half Dollars have been discovered since 2005, and that the most recent find was in late 2013 or early 2014, when an example was purchased as a Punctuated Date 1817!