2001-D Lincoln Cent -- Muled With a Roosevelt Dime Reverse -- MS66 Red PCGS. A mule is a coin struck from mismatched dies. In the long history of the U.S. Mint, if patterns are excluded, a total of perhaps 30 mules are known. Best known of all is the circa 2000 "golden" dollar / quarter mule that pairs a Washington state quarter obverse and a Sacagawea dollar reverse on a Sacagawea dollar planchet. About twenty examples of that mule are confirmed, and they regularly command six figures at auction. The state quarter / Sacagawea dollar mule is ranked #1 in the 100 Greatest U.S. Error Coins reference.

Among a multitude of mule possibilities, the Lincoln cent / Roosevelt dime mule is most likely to occur. The diameters for the cent and dime are similar, and both types have annual mintages that often exceed a billion or more pieces. Nonetheless, mules are great rarities. We know of only seven cent / dime mules:

Probably, the four known 2001-D cent / dime mules were struck on the same day, by the same die pair, on the same press. The dime reverse on the present pumpkin-gold example has a broad rim with raised and recessed elements. The wider rim on the reverse is caused by the comparatively narrow diameter of the dime die. The letters in GOD WE TRUST are lightly brought up, but the remainder of the strike is sharp.

Heritage has auctioned all four known examples of the 2001-D cent / dime mule, and it is interesting to compare images of the four coins. The present lot appears to possess the best eye appeal, because the other three coins display many minuscule toning flecks. The planchet was well prepared, without the tiny trapped gas bubbles that affected the #3 coin from the list above. An important opportunity for the advanced error specialist.

To read the complete lot description, see:
2001-D Lincoln Cent -- Muled With a Roosevelt Dime Reverse -- MS66 Red PCGS....(https://coins.ha.com/itm/errors/2001-d-lincoln-cent-muled-with-a-roosevelt-dime-reverse-ms66-red-pcgs/a/1371-4450.s)

To read a nice CoinWeek article by Charles Morgan, see:
This Rare Lincoln Cent Roosevelt Dime Mule Is Worth $100,000 or More(https://coinweek.com/rare-lincoln-cent-roosevelt-dime-reverse-mule-worth-100000-or-more/)