This mishap occurs when opposing dies collide with each other when a planchet fails to be fed into the striking chamber.  Most complete planchet misfeeds don't result in clashed dies because dies normally don't touch each other in the absence of a planchet.  The dies operate with a safety margin that can be referred to as minimum die clearance or minimum die distance.  These terms refer to the closest approach the dies make to each other in the absence of a planchet.  In order for dies to clash, the dies must have fallen out of adjustment so that their minimum clearance is zero (or less than zero).

    This 1988 cent shows strong clash marks on both faces that persist even after vigorous attempts to abrade them away by mint technicians.  These clash marks are also somewhat misaligned (horizontally) and slightly rotated.