Although no other coin may be similar to a coin with an off-center strike, off-center strikes happen often enough that buyers can choose from many examples each of which varies slightly from the other.Mint error coins can be the result of deterioration of the minting equipment, accidents or malfunctions during the minting process, or intentional interventions by mint personnel. Accidental error coins are perhaps the most numerous and in modern minting are usually very rare, making them valuable to numismatists. Intentional intervention by mint personnel does not necessarily include a deliberate attempt to create an error, but usually involves an action intended to improve quality that miscarries and creates error coins instead. Errors can be the result of defective planchets, defective dies or the result of mistakes made during striking. The planchet, die, and striking (or PDS) classification system happens to correspond with the mintmarks of the three largest U.S. mints, Philadelphia, Denver, and San Francisco. Not all errors fall neatly within the categories. Sometimes design elements are missing from coins because die crevices are filled with grease. Labels used to identify specific categories of errors sometimes describe the cause of the error (die crack, rotated die, clipped planchet). Other errors names describe what the viewer sees when looking at the coin (wavy steps, trails, missing element) while others have names that were adapted for use (mule, cud, brockage). The result is that some errors are known by multiple names. Filled die errors are also known as missing design element errors and as strike throughs. As is noted below under the discussion of missing design element coins, some errors have multiple causes. A rare error that sold for $5462.50 on Heritage Auctions in August 2010 is an undated U.S nickel struck on top of a 1960 5 centavos. Foreign coins struck on a U.S coin planchet or vice versa are very uncommon and hold a high value.