Understanding the rule (All images via Reu, AP)
Real Madrid beat Atletico Madrid in a shootout to advance to the quarter-finals.
During the shootout, Julian Alvarez slipped and the VAR spotted that his left foot touched the ball slightly before he kicked it with his right.
Real Madrid goalkeeper Thibaut Courtois immediately protested, claiming that the Argentine made two touches on the ball.
With only one touch permitted, the penalty, which would have levelled the shootout at 2-2, was chalked off.
As per the rule book, the penalty is invalid if the taker accidentally makes a second contact—either with the same foot, the other foot, or any other part of their body.
The kicker must not play the ball again until it has touched another player.