Chess Opening Theory/1. e4/1...e5/2. Nf3/2...Nc6/3. Bc4/3...Nf6/4. Ng5/4...Bc5/5. Nxf7/5...Bxf2/6. Kf1/6...Qe7/7. Nxh8/7...d5/8. exd5/8...Nd4

8... Nd4
If it were Black's turn right now, they would love to play Bg4 to effectively trap White's queen.

There are two common mistakes when dealing with this threat. One is 9. Kxf2, but Black still has 9...Bg4!. If 10. Qe1, Black has 10...Nxc2 forking the rook and queen. If 10. Qf1, Black has the devastating Ne4+; Kg1 is White's best response and Ne2+ loses the queen. 10. Qg1 is somehow worse, giving Black checkmate in 4 moves.

The other mistake is 9. c3, still allowing 9...Bg4! because after 10. Qa4+ Nd7 11. cxd4 Bxd4, White faces the unavoidable Qf6+, with too many threats to deal with. 11. Kxf2 similarly deals with the same issue of Qh4+. In fact, White's only advantageous move is the clever 9. d6, deflecting the Black queen away from the knight.