The 1999-S Proof Close AM Lincoln cent was accidentally struck with a reverse die intended to strike circulation strike coins only.
All Proof Lincoln cents from 1993 to 2009 should have the letters AM of America separated from one another. However, some Proof 1998 and 1999 Lincoln cents have been discovered with the letters AM of America Close to each other and almost touching, which indicates that it was accidentally struck with a mint state die as the in 1999 the Mint struck circulation strike Lincoln cents with a Close AM design.
There are possibly several thousand examples known for this variety and the Close AM variety can be seen with the naked eye. Any example of this variety in any grade will command hundreds of dollars and up to thousands of dollars in really high condition.