PCGS 1999 Lincoln cent dual date double-strike, struck on a 1996 Lincoln cent. This is a 1999 cent struck on a 1996 Lincoln cent, with both dates plainly visible with magnification. 

The error occurred when a 1996 cent somehow made it into a press striking 1999 cents and was struck. There are several possibilities, which the most likely include a Mint worker dropping a 1996 cent out of his pocket in 1999 on the ground, and another worker throwing it into a bin of cent planchets, which were later fed into a press and struck. Another possibility is a worker threw a 1996 cent into the bin intentionally, knowing it would create a "wild overstrike."

Another possibility is that a 1996 cent was struck, became lodged in some part of the press or fell on the ground and was kicked underneath some machinery, then in 1999 it became dislodged and made its way into the press that, three year later, was striking 1999 cents and was struck. Any of these, and other scenarios, could have resulted in this coin being produced. 

Extremely rare, and actually far more rare than a dual date double-denotation, which is how almost all dual date coins come. Having a dual date on the same denomination is far more rare, and this is probably only  second or third example we have had of a dual date on the same denomination. 

MS63 Red/Brown with some brown toning but with red, lustrous surfaces shining through the toning.