Definition: Tiny raised circles, semicircles, and occasionally spirals that are most often found in or near the center of the design. The imperfections can be found on either face. First recognized in 2004 cents, these die imperfections have now been found as far back as a 1965 cent and as recently as a 2015 nickel. Other examples are known from 1988(P), 1999(P) and 2010(P) cents. The cause of these imperfections is unknown. While some have compared them to concentric lathe marks, it’s doubtful there is any connection.
2015-P Jefferson nickel with circle on the reverse
Seen above is a less than perfect circle intersecting Lincoln’s statue on the reverse of a 2004-P Lincoln cent.
The exact cause for these anomalies is unknown. A conjecture has them associated with the finishing lathe. Seen above is a semi-circle intersecting Lincolns statue on the reverse of a 1993-P Lincoln cent.
The images above show a 1988-P obverse Lincoln die with a raised circle on Lincoln’s jaw line. The black arrow points to a “struck through a foreign object”