It occurs when the dies are struck together without a blank in between. You'd have thought it would be catastrophic but it seems that they survive pretty well, leaving just an impression of one on the other. Clashed dies are extremely numerous on pre-Jubilee Victorian coins, especially the smaller denominations. This is an 1840 farthing (2 prong trident) variety that is struck from clashed dies, this variety always has the clash evident and it is very clear. That is the best example I have seen. I have an example.Colin Cooke stated he had only seen 3 in 25 years. There was one on Ebay the other night and several of us knew what it was.The seller must of been shocked as it was a 99p coin without the clash. This is mine. http://www.omnicoin.com/coin/947669 I'm happy with mine and have no intention to improve it.