Cleaning Damage: Improper cleaning methods can leave coins with a polished or overly bright appearance, which is generally undesirable. Harsh cleaning can also result in scratches, loss of original luster, or even wear down the coin’s relief.
Physical Alterations: This includes any form of deliberate modification, such as drilling, engraving, or bending. Such alterations are easily identifiable and typically render the coin of little interest to serious collectors, except in cases where the modification has historical or cultural significance.