The 1998-S Proof Close AM Lincoln cent was accidentally struck with a Mint State die.
All Proof Lincoln Cents issued from 1993 to 2009 should have the AM of America separated from one another. However, some proof 1998 and 1999 examples have been found with the letters AM of America Close to each other or almost touching, which indicates that it was struck with a mint state die.
There are less than 100 examples known and the variety can also 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 high PR69 Deep Cameo condition. As of 2009, there are no PCGS PR70 Deep Cameo coins known with this variety, despite there being dozens of regular 1998-S cents graded in PR70 Deep Cameo.