Why did the 1979 s proof dollar become an error coin?

Can you elaborate on why the 1979-S proof dollar was classified as an error coin? Was it due to a misprint, a deviation from the standard design, or some other manufacturing flaw? Was it a mistake that occurred during the minting process, or was it a deliberate change that was later deemed inappropriate? Understanding the specifics behind this error coin's classification would help us appreciate its rarity and significance in the world of coin collecting.