Croatian midfielder Luka Modric did not notice that he was due to be withdrawn as part of a double substitution by Real boss Zinedine Zidane, leading to a brief moment when Real had 12 players on the pitch.
The extra man may have come in useful for Real, who fell to a late defeat at the Barnabeu thanks to Antonio Sarabia's late header from an Antonio Barragan cross.
The incident occurred in the 71st minute when Zidane sent on Lucas Vazquez and Borja Mayoral in place of Marcelo and Modric. However, the Corat didn't notice that he was due to be withdrawn and remained unmoved.
Referee Mateu Lahoz then stepped in just as the ball was about to be put in play to point out the mistake and avoid what would have been quite the scandal.
The desperation of 'Los Blancos' to seal the victory was there for all to see, but it was not enough to force a result with Real's defeat now meaning they find themselves seven points behind table-topping arch-rivals Barcelona after five games.