Technically a variety and not a mint error since several varieties of die cracks, like the Speared Bison, have been discovered that are exactly alike. Since this is a die issue or accidental modification from stress then it becomes a variety. It should be noted that not all die cracks are varieties.
A die crack occurs when the stress on the die begins to cause fractures in the die face, these die cracks are not deep but are the beginning of the end for the die. Die cracks rarely add any extra value to a coin, but can be used as markers for know varieties called PUPs (Pick Up Points). In some cases when two or more coins were found with the very same die crack they have been attributed as a variety.