Mint gunk(grease, metal dust, etc) can clog the fine details of a die, causing a loss of detail on the resultant struck coin(s). Grease is the most commonly found struck through and will typically appear as a thin layer of softened detail. Since grease can flow, its effects will usually vanish over a series of strikes without the die needing any maintenance.
Grease mixed with metal dust can form a hardened concretion that will completely clog a die in a particular area. Occasionally, a clog of this type in a letter or number will dislodge and land on the next planchet to be struck. The clog is then pressed into the newly struck coin, forming a perfect impression of that letter/number. This is known as a dropped letter error and is a fairly desirable error.