Part V: Planchet Errors:

Alloy Errors: 

Slag inclusions:

Definition: The earliest point at which foreign matter can insert itself into a coin is when the molten alloy is poured into a mold for the ingot. During smelting, unwanted contaminants float to the top of the molten metal in the form of slag. This slag is supposed to be skimmed off before the alloy is poured into the mold for the ingot. If small pieces of slag remain behind, they can end up in the ingot and persist there through all the subsequent steps of the minting process – rolling, blanking, upsetting, and striking. 

A 1979 cent with a large slag inclusion that extends through the entire thickness of the coin.  The original nodule was much larger, but about half of it fell out after the strike.