Thanks for checking.
You want to know a funny one?

New game -> D2 D4 -> GO -> it will probably say D2xD4 (empty square takes your white pawn). And indeed, after verifying, the white pawn is gone.

This is because the move it generates, is from the board representation before the user made their move.

Ah, Berger already replied.
But I disagree that a legal move is not a bug. Black moving white's pieces is still an illegal move.