Engraver errors are among the most popular of all varieties. As opposed to errors in the minting process (for example, off-center strikes, blank planchets, brockages, etc.), engraver errors are simply goofs made by the person preparing the die. Examples of engraver errors include overdates, misplaced dates, an incorrect number of stars, misspellings, etc.). One of the more interesting engraver errors among 1794 Large Cents is the No Fraction Bar variety. In this case, the engraver omitted completely the dividing bar of the 1/100 fraction on the reverse. This error occurred on only one variety of the year (Sheldon-64), so the engraver gets some credit for getting it right on the many other varieties of 1794.
The No Fraction Bar is a fairly scarce variety, and one that is desirable in virtually every grade. One of the best certified examples is a PCGS MS64RB that shows nearly 25% of the original red color -- a very unusual situation on any early U.S. Large Cent.
By the way, the name for the dividing bar in a fraction is "vinculum."