A
Die Cud is when part of the die breaks off the outside edge of the die:

With that missing area on the die, it creates a void and whatever metal is left on the planchet during the strike will fill in that area first. Because the opposite die has no pressure forming the normal opposite devices, that area on a
Cud will be weak, showing just a partial affected area.

Note the weakness on the opposite sides of the where
Cud is:



