Major mint errors on U.S. gold coins are extremely rare. There are only a few known that are double or triple struck. One phenomenal example is this double stuck 1904 $20 rotated 180 degrees in the collar. Considerable detail remains from the initial strike and is visible under the portrait on the obverse and the eagle on the reverse. This exciting mint error was in my personal collection and has been authenticated and certified by NGC and BYERS COLLECTION is noted on the insert.
A different double struck 1904 $20 gold piece with slight rotation sold a few years ago for $100,000. It was authenticated and certified by PCGS MS 63. I bought it and placed it with a customer and it is featured in my NLG Award winning book, World's Greatest Mint Errors on page 57.