"A player must capture as many pieces as possible." What happens if he doesn't?
As per the the Official FMJD Rules for International Draughts, if the opponent notices that you have not captured as many pieces as possible, they may decide whether the move stands or if it must be taken back and made again.


  5.4. If a player has committed one of the following irregularities, his opponent has the right to decide whether that irregularity must be rectified, or on the contrary, must be allowed to stand.


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  5.4.8. Capture by a piece, of less or more than the number of possible pieces to capture;