Die clashes occur when both hammer and anvil dies (containing the design for the 'heads' and 'tails' of the coin) hit each other with no coin blank (planchet) present.
The result can be dramatic, as in the case with this cent. Both dies transfer design elements from each other onto the opposite die, due to the tremendous tonnage used in coining presses. Subsequent coins struck will exhibit these errors as incuse design elements from the opposite side. This continues until the errant designs are worn off, or polished off with an abrading tool.
Images taken by myself, of this cent I purchased on eBay.
