Die defect

A die defect is a unique and unintentional flaw in a coin die and is created through excessive use or polishing of the die. A die bearing such a defect is occasionally referred to as a defective die. Generally, and depending upon the magnitude of the defect, coins that are produced from these dies are considered error coins. Also, the term encompasses a wide variety of design errors that were engraved into the die originally and were slipped into circulation before the incorrect design was discovered.
Types.
Die crack.
A die crack occurs when a die, after being subjected to immense pressure during the minting process, cracks, causing a small gap in the die. If this damaged die continues to produce coins, the metal will fill into the crack, thus revealing a raised line of metal in the finished coin. Specimens with more prominent die cracks can command a high premium and are valued greatly by some collectors. However, less obvious errors are quite common, especially in the 50 States Commemorative Quarter Program, yielding a lower value.
Cud.
A cud is a variation of a die defect in which the coin bears a raised portion of metal. Unlike a die crack, this unintentional "bump" in the coin is caused by a dent or gouge in the die, therefore allowing the coin to fill into the gap during the minting process. A very minuscule cud can look like a small chunk of metal that was somehow fused to the coin. Some numismatists disagree that these small disturbances, although produced in the same way as a cud, can qualify as one. A prominent cud looks like a blank section of a coin, usually toward the rim of the coin.