This dime planchet had a second planchet directly on top of it (and centered in the collar) when it was struck. The end result was 2 coins; one with only an obverse strike, and the second with only a reverse strike. Notice the presence of a rim on the unstruck side, which is one of the best clues to look for on a genuine one-sided coin, along with checking its weight which should be within tolerance. In addition, notice how a ghostly design transfer from BOTH dies appears on it. This is a creepy, cool, and rare error type to have an in-collar, dual planchet strike with a full uniface strike. Graded MS64 by NGC. Enjoy!
No, no, it's clearly a prototype overstruck on a dime planchet as part of the Mint's production of foreign coins with fuzzy elephant merry-go-rounds on the reverse. You just need a better microscope to prove it. Oh, right, that guy's gone now. Phew.