Is It a Rare Error or Just Damage? Understanding Post-Mint Damage 

The distinction is simple and comes down to timing:

• A Mint Error is a mistake that happens during the manufacturing process at the U.S. Mint. These are faults in the die, the metal planchet, or the striking of the coin. Because they are genuine production mistakes, they can be quite rare and valuable.

• Post-Mint Damage (PMD) is any damage that occurs to a coin after it has left the Mint and entered circulation. These are not errors and, in nearly all cases, they reduce or eliminate any collectible value a coin may have had.

Coins that have been buried in the ground, exposed to chemicals, or left in a damp place can develop corrosion, pitting, and strange colors. This isn't a "toning" error; it's irreversible damage to the metal. 

A coin stuck in a commercial tumble dryer gets rolled and heated. The edges get hammered and rounded, often making them thick and smooth, while the surface details get worn away. 

Sometimes people alter coins on purpose. This includes coins that have been bent, drilled, or squeezed in a vice. "Spooning," where a coin's edge is repeatedly tapped to form a ring, is another common example.