NGC 1927 double eagle with a severe planchet crack, and which extends half way across the coin on both sides starting at the 8 o'clock position. When held up to a light source, you can even see light through about 1/4 of the crack! This is a huge split for a gold coin, and is especially rare on a large size gold coin. The quality controls on gold coins were extremely high (as they still are to this day), and even the most minute defects caused the coins to be rejected. A large planchet defect such as what this coin has, is all but unheard of on a $20 gold piece. This is one of the few examples of a cracked planchet we have seen on any U.S. gold coin.
We can say that this is the most dramatic Saint Gaudens $20 error we have ever heard of in a coin series on which virtually no errors of any kind are known, other than the most minute strike throughs, disc clips and similar minor errors. Major errors are nearly unheard of for the series, and this is the most dramatic example we know of.
The planchet crack occurred due to impurities in the gold alloy, which caused the metal to not bond together properly in the area of the crack, and when the coin was struck, the metal simply did not bond together but remained separated. Because gold is such a valuable metal, special care was taken then (as it is now) and cracks such as this are incredibly rare on large U.S. gold coins. Other than one other very minor example, this is the only other known to us of a cracked planchet, and it is far and away the most dramatic, extending half way through the coin.