Illegal positions

Conventionally every chess problem should have a legal position. Naturally then, an illegal position is a position that cannot be reached by a legal game. For instance, a position in which one side has more than 8 pawns, or has both White and Black Kings in check, is illegal.

A composer of chess problems can have fun creating these...  

Diagram - White has just moved a pawn from d7 to d8 and promoted to a knight to checkmate the king, but the move is illegal. Why ?

👍 Read - Who's the Goof ? #5 to discover why
                           solution on page 8

* Warning - problems like this can do your head in !