Errors and Varieties
One of the principal objectives of the minting process is to produce coins which are identical for any given denomination and year. If coins were of random design then there'd be no way to distinguish a forgery from a genuine coin and public confidence would be undermined to the point where coins would be useless as Currency.
The cost of the raw materials is typically less than the face value of modern coins and so anyone with access to those materials and the machinery to process them would be able to issue tokens purporting to be coins. With uniform designs, the forger's task is more difficult in that the official design must be replicated exactly; this is something that mints do but which is not so easy for a forger to achieve.
Despite the objective of complete uniformity, variations do occur. From a numismatic standpoint this is quite fortunate because a completely uniform currency is rather boring.
Varieties can originate from just about any stage of the minting process but can be divided into two groups, errors and die varieties.
Errors are the result of defects in planchets (blanks) or of misadventure during the coining process. Most errors are filtered out by post-strike inspection but occasionally an error coin gets missed and passes into circulation. Error coins tend to be unique and many collectors value them for their rarity.
Die varieties, as the name suggests, arise from differences in the dies used to strike coins of a particular denomination and year. A common variation is the addition of a mint mark to a basic design to designate a particular mint as the source of a coin. Mint marks are small, do not really affect the overall design and so do not diminish the mint's goal of uniformity. Other variations can be much more subtle.
Some die varieties may be classified as errors and may arise during die or punch preparation or during minting.
The latter group includes die filling (where metal from a planchet remains in an incuse portion of the die, lessening or obliterating all or part of a relief feature in subsequent strikes), die breaks (where a relief portion of a die breaks off, altering the shape of a feature in subsequent strikes) and die cracks (where a die starts to disintegrate under the stress of repeated strikes and creates an irregular, raised feature on subsequent strikes).
There are many other faults which can occur. A die can be damaged during coining. For example, if a blank fails to feed into the coining press, the obverse and reverse dies can come into direct contact with each other. This is known as a die clash and affects coins struck thereafter until the damaged die is retired. A hard inclusion in a blank can damage the surface of a die, causing a "blob" or "egg" on subsequent strikes.
Although the minting faults described above are technically errors, the fact that the faults are faithfully reproduced on many coins elevates them to the status of varieties, albeit minor ones.
Major die varieties arise from fundamental differences in the dies used to strike coins. Sometimes there are different master dies used for the coins of a given year, and sometimes there are variations in the working dies produced from any one master die.
Whatever the origin, die varieties are propogated onto thousands or millions of coins. Unlike errors, die varieties represent systematic variations and are the subject of considerable interest.
Always be on the look out for errors and varieties. You never know what you might find!!