I'm not sure if this is even an error. Possible struck through some debris? or a bad planchet to begin with?
Possibly a lamination error before the coin was struck. The detail in the letters seem to strong to have been struck through. Hard to say......every coin has a unique story.
I think it means when a piece of debris like a piece of cloth or a piece of metal is between the planchet and die when struck . I'd say bad planchet .
this is a lamination error like others have said , it is quite common in the silver alloy war nickels
I think lamination too. The crack above Monticello and going into the dome is a lamination crack and the voids on Monticello would be detached lamination.