🧠🧠🧠 Explosive Queenside (Illegal Move Exercise)

Illegal Move Exercise: The audio will lead you through a series of moves from the beginning of a game. Somewhere in there, a player will make an illegal move but the game will continue anyway. Find the illegal move.

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PGN for today's exercise:

1. b3 a5 2. Nc3 e6 3. a4 b6 4. d4 b5 5. Be3 bxa4 6. bxa4 d6 7. Qd2 Bd7 8. g3 Nc6 9. f3 Na7 10. Bg2 Nb5 11. Nh3 Nxc3 12. Bxc3 Nf6 13. Bxa5 Be7 14. O-O O-O 15. e4 Qc8 16. Nf4 Qa6 17. Bc3 Bxa4 18. Rfb1 d5 19. Bb4 Bxb4 20. Rxb4 Qc6 21. e5 Nd7 22. c4 dxc4 23. Ng6 hxg6 24. f4 Qa6 25. Bxa8 Rxa8 26. Rbxa4 Qxa4 27. Rxa4 Rxa4 *

And the answer is...
White’s move 12… Bxc3 is illegal as White’s dark-squared bishop is on e3 and cannot capture Black’s knight on c3.