Definition: A retained struck-through error is a piece of foreign matter that is struck into a coin and that remains embedded in its surface.  Such errors can be difficult to distinguish from rolled-in errors.

A retained, struck-through error (otherwise known as a “struck-in” error or an “embedded struck-through”) will usually be demarcated from the rest of the coin by a narrow fissure.  If it is a sizable piece of debris, it will push the coin’s weight beyond normal tolerances.

Almost everything imaginable has been found struck into coins.  The most common subjects are pieces of scrap coin metal.