When a planchet fails to enter the coining chamber and both the hammer die strikes the anvil die, imparting design features of the opposing die into each die face. When a planchet does enter the coining chamber it is struck with the clashed dies and will show details of the obverse on the reverse and visa versa.
Die clash values are minimal at best and are common enough that they add very little value to a coin. The coin in the above image was found in a roll and graded MS66 by PCGS as a Die Clash and it sold for $75. The grading fees alone were almost $50 so not much room for premium. Older type silver coins like three cents, dimes and half dimes are known for die clashes.