It's actually not an "error" It is simply a die variation. Either VAM 9 (
http://www.vamworld.com/1878-S+VAM-9) or VAM 21 (
http://www.vamworld.com/1878-S+VAM-21)
There is extensive cataloging of the different combinations of obverse and reverse die pairings... each die used in the minting process will have ever so slight variations from the previous die used. Each year is different, so VAM 9 for 1878-S is not the same as VAM 9 for a 1880-O.
There are 3 major categories of VAMs.. TOP 100, HOT 50, HIT LIST 40 These are the most widely collected varieties of Morgan Dollars and will be noted on the holders of PCGS and NGC if you pay for the additional service. Some VAMs will bring multiples higher than that of their more common varieties. A recent "scarface" 1888-O was sold at auction for over $5000. It was graded MS61. It's more common counterpart in the same grade sells for $54.
Sorry, but what you have is not a widely collected variety in today's market. I'm not sure why NGC would have noted the coin as an "error" But, I can say that VAM coins are a relatively fresh market, even though the varieties were being discovered decades ago. It could be that the coin was sent to NGC before the industry got a better grasp on the market for these coins.
Brick and mortar shop: buy, sell, and trade anything precious- coins, bullion, scrap, jewelry, gems, etc.
http://marketharmony.netFollow me on Twitter- @MarketHarmony