These coins would likely be special editions or mint errors that were accidentally released into circulation. For example, certain coins may have unique designs, misprints, or errors that make them exceptionally valuable to collectors.
The $15 million valuation for each coin suggests an extraordinarily high value, far beyond what is typically seen even for the rarest coins. Such a valuation would likely only be achieved through a combination of extreme rarity, historical significance, and collector demand.
The term "still in circulation" implies that these coins have not been removed from circulation and are still being used in everyday transactions. This is highly unlikely for coins of such extreme value, as they would likely be quickly identified and withdrawn from circulation by collectors or individuals aware of their worth.
Any claim of coins being worth $15 million each would need to be thoroughly verified by numismatic experts and authenticated through reputable channels. Without proper authentication, it's difficult to ascertain the true value of such coins.