Example below. The FEN is valid but the move is not. An internal board state is created which is impossible (a knight pulled out of thin air that was not present on board).
position fen r4rk1/1R5p/3p1q2/p1pP4/P7/7P/6P1/1R1Q3K b - - 0 24 moves b8e8
fen
r3nrk1/1R5p/3p1q2/p1pP4/P7/7P/6P1/1R1Q3K w - - 1 25
Example below. The FEN is valid but the move is not. An internal board state is created which is impossible (a knight pulled out of thin air that was not present on board).
position fen r4rk1/1R5p/3p1q2/p1pP4/P7/7P/6P1/1R1Q3K b - - 0 24 moves b8e8 fen r3nrk1/1R5p/3p1q2/p1pP4/P7/7P/6P1/1R1Q3K w - - 1 25