Flan cracks, from the edge of the coin, due to stress during striking, are very common on ancient coins. A flan crack that is open into a V-shape may be called a flan split. Flan cracks are least common on gold coins, due to the greater malleability of that metal, and most common on silver coins. For some types, flan cracks are rare and for other types almost every example will have multiple cracks.
Some flan cracks significantly detract from appearance and greatly reduce value, others are insignificant and don't impact value at all. A flan crack on a type that is rarely cracked will have a larger impact on value than a crack on a type that is rarely cracked. Short small cracks usually have very little impact on value. Cracks that extend more than half way across a coin will almost always have a significant effect on value. A large wide flan split will have a larger impact on value than a small thin closed crack. A crack that protrudes deep into the coin and detracts from the design will have a larger impact on price than a crack that is only on the edge or in a field. A flan crack on an otherwise perfect coin will likely have a high impact on value, but a flan crack on a worn and otherwise unappealing coin may have little impact on value. The impact of a crack on price will mostly correspond to the diminished eye appeal resulting from the crack.
Flan cracks and splits usually do not indicate a coin is fragile or risk of breaking, but they may in the case of crystallized silver coins. If a coin is cracked, crystallized and fragile, reputable sellers will note the fragile condition in auction and sales listings.
A few examples of flan cracks are photographed below: