The most significant error variety of the 120,000 minted we’ve seen to-date. It appears to have been created by some foreign object that fell onto the obverse die or was attached to the planchet before striking commenced. See recent commentary from Coinweek, the online numismatic magazine:
The 2016 Mercury Dime Centennial Gold Coin is beautifully struck in .9999 fine 24–karat gold, and features Adolph A. Weinman’s original Winged Liberty design. This coin is commonly referred to as the “Mercury” Dime because of Liberty’s resemblance to the Roman god. Each coin contains one–tenth troy ounce of 24–karat gold, symbolic of the coin’s denomination of one dime, and is struck with a business strike finish.
The obverse (heads) features a portrait of Liberty facing left, wearing a winged cap, with the inscriptions “LIBERTY,” “IN GOD WE TRUST,” “2016” and the artist’s initials, “AW.”
The reverse (tails) includes a Roman fasces and an olive branch, symbolizing America’s military readiness and desire for peace. Inscriptions are “UNITED STATES OF AMERICA,” “E PLURIBUS UNUM,” “AU 24K,” “1/10 oz.,” and “ONE DIME.”