Ron Guth:
The "Comet" variety of the 1807 Large Cent is so-named because of a flaw in the left obverse field that connects the rim with Liberty's hair just above the hairbow. This flaw is unique to the Sheldon-271 variety and does not appear on any other 1807 obverse. However, the flaw itself is not diagnostic of the S-271 variety, as early states of the dies are perfect and do not show the flaw. In actuality, this die contains numerous flaws that appeared and worsened asa the die aged. The additional flaws included numerous die clashes, a parallel, raised line in the right obverse field in front of Liberty's face, and a horizontal crack across her bust at the throat level. This is a very popular variety because of the obvious flaw and the numerous die states.
Though this is a fairly common variety, it is difficult to locate in high grade, especially Mint State. As of March 2011, PCGS had certified only eight Mint State examples, five in MS-63BN, one in MS64BN, and two in MS65RB. None have been grade finer and full Red examples are known.