NGC 1973-S proof Kennedy half with strong clashed dies obverse and reverse. The eagle's outer design and the rays are plainly visible on the obverse, and Kennedy's head can be seen on the reverse.
Proof dies are very scarce with strong die clashes, since the dies are quickly repaired or retired when they develop die clashes.
Clashed dies occur when there is no planchet between the dies, resulting in the two dies clashing together and transferring some of their design to the opposing die.
PF69 Ultra Cameo an especially high grade example.