🧠🧠 *error* (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. d4 Nf6 2. c4 e6 3. Nc3 Bb4 4. Bd2 c5 5. e3 cxd4 6. exd4 d5 7. Nf3 O-O 8. Bd3 dxc4 9. Bxc4 a6 10. O-O b5 11. Bb3 b4 12. Ne2 Bb7 13. Ng3 Bxf3 14. Qxf3 *

And the answer is...
Black's move 11... b4 is illegal because the square is occupied by the black Bishop.