During the 1st year of issue for the Morgan Silver Dollar, they made a small mistake at the Mint regarding its design -- specifically, they had the wrong number of tail feathers on the eagle.
Media pundits, and other prominent individuals, led a campaign to have the coin's design changed because the current version had eight tail feathers on the reverse and they believed that eagles actually had an odd number of feathers -- though, this would eventually be disproven decades later.
As a result, the U.S. Mint only struck the 8-Tail Feather design for 11 days before it was changed to the 'corrected' 7-Tail Feather variety. When the dust settled, they had only struck 749,500 coins with this design, making them some of the most sought-after Morgan Dollars in the entire series.
We have a small number of these 'original' 1878 Morgan Dollars struck at the Philadelphia Mint, with the 8-Tail Feather design, that have been certified by NGC as being in stunning MS63 condition.
In this grade (as of 01/16/2024), NGC has only certified 9,161 coins in MS63 or better condition with the 8-Tail Feather strike -- that breaks down to just 1 in every 82 coins struck.
That is just 1.2% of the entire 8-Tail Feather Morgan Dollar mintage.