This occurs when there is a jam or stoppage of the flow of blanks to the press. There is a micro-switch which detects the lack of blanks being fed which then in return stop the press to prevent damage to the dies. If the jam occurs in a way that the sensor does not detect the jam, the press would continue to operate as if the blanks were being fed. At this point, the obverse die would make an impression on the reverse die and the reverse die would make an impression on the obverse die. Once the jam is corrected, each and every coin that is struck will show the "clashed die". If the clash dies are caught, the dies will be removed from the pres and sent back to the engravers for repairs. The repairs usually consist of heavy polishing to the dies to reduce the effect of the clash marks. After all repairs are completed, the dies are sent back to the mint floor and put back into service.