|image 1: the coin|
My friend just found this coin in his loose change. Currently do not know what had caused this error but I am pretty sure that this is not a post mint defect (PMD) coin.
Currently, some collectors think that this is a Multiple Struck Coin.
I wondered this is a Multiple Struck Coin.This coin received many times of strikes on the (almost)same spot when it was stuck in the mint press and could't be ejected.The continuous striking caused higher rim to be formed,and caused the lettered edge to deformed.The impact on multiple struck on the same spot may caused high temperature and and resulted the coin, especially smaller denominations to become darker in colour. You may find that the center of the coin is thinner,due to the impact of continuous struck,the metal was pushed to the edge,hence,form a higher rim. (Dickson Niew)
And it could be a coin which was not struck by the third collar die. (Nu Coins and Notes)