There are two types of errors that result from the minting anomaly known as a capped die. A capped die is caused when a coin or planchet sticks to the hammer die. In turn, the coin/planchet that sticks to the die becomes a die itself, striking coins until it eventually falls off. Coins struck by the capped die will have two sides that bear the same design, only one side (the one struck by the capped die) will be mirrored and distorted. The design on the side of the coin struck by hammer die in a capped die strike can range of slightly distorted to completely gone, so one side almost looks blank.

A coin that is struck in such a way is known as a brockage (a term that applies to a coin that is struck underneath or on top of another coin/planchet). The other type of anomaly that results is the die cap itself. While capped die strikes are rare and highly sought after, the capped die itself it many times rarer. The reason is because a capped die strike will always be normal sized (if it is only struck once and in collar), but the die cap is several times the thickness of a normal coin and is actually in the shape of a thimble! For that reason, it is possible to find a capped die strike in a coin roll, but absolutely impossible to find a die cap in a roll.

You many wonder how such an egregious error made it's way out of the mint. There are two possible answers to that. The first is that it could have ended up in a bag of coins to be sent the Federal Reserve Bank. The second is that it was intentionally removed from the mint building by an employee. While the latter is illegal, things like that are rarely prosecuted because proving the mint employee actually stole anything is very difficult.

Whatever the means of the coins escape from the mint, the error is extremely popular among error collectors and presents a huge challenge to anyone looking to obtain one. That, and the coins dramatic appearance make it something very interesting to study but very difficult to own. On the other hand, a capped die strike (or brockage if you prefer) is much more available, albeit quite pricey. I have included a diagram that should help explain how the hammer and anvil dies work.