Steel “staples” are sometimes found embedded in coins.  They are not really staples at all. Instead they are believed to be detached bristles from wire brushes used to clean encrusted gunk from the surface of dies.

This 1966(P) Lincoln cent was struck with a “staple” resting on top of the planchet.  The “staple” has been retained, making the specimen much more desirable than a coin with merely the impression of a “staple”.

Images are courtesy of Heritage Auctions.