A very unusual variety surfaced in 1990 when a few dozen examples of the 1990 Lincoln proof cent appeared without its requisite S mintmark. As the months turned into years, only a handful of other 1990 no S proof cents surfaced, proving this variety to be an especially rare error indeed. In an era of tight quality control at the United States Mint, it seems unfathomable that an error of this type, one in which a San Francisco Mint proof coin is missing its mintmark, could have ever been created, let alone was able to escape the mint. Yet, it did.

In all, fewer than 200 examples of the 1990 no S proof Lincoln cent have been discovered. On rare occasions, one still turns up in an unsearched hoard of 1990 proof sets. Whether or not many more exist remains to be seen, but it is without a doubt one of the rarest and most significant of all modern proof varieties and demand remains strong for this sensational error.