At some point during the life of the die, something occured to cause a triangular piece of metal to be raised up from the surface of the die near the edge. This excess metal, the opposite of a die chip, causes a small triangular indentation in the coin. This must be a defect on the die as the exact same feature appears on numerous coins.
Excess Die Metal EM is commonly paired with Obverse Die Crack 2005-3 and Reverse Die Crack F1.