Black mixed systems and paid the ultimate price. He forgot White played 6. Bc4 on the 6th move instead of 6. d4. After 6. d4 d6 7. Nd3, 7...Nxe4 is totally playable. But when White plays 6. Bc4, 6...Nxe4?? is a blunder because f7 is hanging. After 6. Bc4, d5 is forced.