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. e4 e5 2. Nf3 Nf6 3. d4 exd4 4. Nd5 Bc5 5. Bc4 0-0 *

And the answer is...

White's move 4. Nd5 is illegal because the Knight cannot jump there from f3, and the other Knight is still on b1.