The 1990 “No S” Proof Lincoln Cent is one of several occurrences when a proof coin was struck at the San Francisco Mint without the “S” mint mark. Other examples include the 1968, 1970, 1975, and 1983 “No S” Proof Roosevelt Dimes. Generally, only a small number of such mistakes are produced and released, which makes for big premiums for discovered examples.
The mistake for the 1990 Proof Lincoln cent occurred when the Philadelphia Mint, which makes the dies used by the San Francisco Mint, failed to put the mint mark on one of the Lincoln Cent obverse dies. As a result, one production run of approximately 3,500 cents were struck with “No S” mint mark. These coins represent a rarity that is in steady demand, with prices into several thousand dollars.
taken from: http://lincolncents.net/