We call them "partial incuse errors". Did't found the right english word for it...maybe: die clash
I make an article over this type of error (in flemish).
But in short.
In this case you have two dies. By error these two dies clash against eachother without an blanco planchet in between them. Marks of the first die are visable on the other die. So when an other coin is minted you see these mark on the coin in mirror image.
mvg, Laurens Aernout
Catalogus van Belgische Numismatische Uitgiften #CBNU