Real Madrid continued the curse on city rivals Atletico Madrid in the Champions League with a controversial penalty shoot-out that brought a VAR intervention.
The holders advanced to the quarter-finals, where they will play Arsenal , on a night of drama at the Metropolitano that will have enraged the hosts and manager Diego Simeone .
Real Madrid have beaten Atletico Madrid in five consecutive Champions League meetings , including the 2014 and 2016 finals, a semi-final, a quarter-final, and now a last-16 clash.
This one went to penalties after Conor Gallagher 's goal inside the opening minute at the Metropolitano cancelled out Real Madrid's 2-1 win at the Bernabeu last week.
Real Madrid had the best chance to win the tie on the night but Vinicius Jr blazed his penalty over the crossbar in the second half - with the tie advancing to penalties after extra time was goalless.
Real Madrid went first and scored their first two efforts, with Kylian Mbappe sending the Jan Oblak the wrong way and Jude Bellingham also scoring.
Alexander Sorloth had scored Atletico's first penalty before the ball was passed to Julian Alvarez , the former Manchester City forward and Argentina's World Cup winner.
Alvarez slipped, however, as he ran to the ball, sending his kick high. But Alvarez seemed to have luck on his side and it flew under the bar and into the roof of the net.
Alvarez slipped and was ruled to have touched the ball with both feet (AP)
The following moments brought controversy and confusion. Before Fede Valverde could take Real Madrid's next penalty, referee Szymon Marciniak stopped the play for a VAR review.
The VAR checked to see if Alvarez touched the ball twice after he slipped. While initial replays broadcast around the world were not conclusive, it was ruled that Alvarez touched the ball with his left foot after he slipped, before kicking with his right.
The VAR recommended that the penalty should be disallowed and the VAR decision confirmed: "Atleti player, No19, double touched the ball when the penalty kick was taken."
Referee Szymon Marciniak gestures as the second penalty kick for Atletico de Madrid, taken by Julian Alvarez, is ruled out following a VAR Review for an improper kick (Getty Images)
However, while referee Marciniak held up two fingers to signify a 'double touch', the decision was not initially broadcast to the stadium, with Atletico fans celebrating even though they now trailed 2-1.
A better angle, from low down and behind the goal, emerged roughly an hour after full-time and showed that Alvarez's sliding left foot had indeed nudged the ball onto his right with his instep after he had slipped.
Afterwards, Real Madrid goalkeeper Thibaut Courtois even claimed that he "sensed" that Alvarez had committed an irregular kick and alerted the referee.
"I sense that (Alvarez) touched the ball twice and I told the refere," Courtois said. "It's not easy to see and it's bad luck for them."
Valverde scored his penalty to put Real Madrid 3-1 up. Even when Angel Correa scored to make it 3-2, many Atletico fans in the stadium seemed to think that they were level.
Atletico were handed a potential reprieve, however, when Oblak saved from Lucas Vazquez. But the hosts couldn't take advantage as Marcos Llorente smashed his kick against the bar.
That meant Antonio Rudiger had the chance to win the tie for Real Madrid. He went left and although Oblak got his hands to it, the ball squirmed through and creeped over the line.
Rudiger scored the winning kick (Getty Images)
Real Madrid players celebrate (Getty Images)
Real Madrid won the shoot-out 4-2, keeping their bid of a 16th Champions League alive. Diego Simeone led his Atletico players around the Metropolitano, gesturing wildly to the Atletico fans.
"I've never seen the VAR called to review a penalty in a shootout... Never!" Simeone, who said he had yet to see an image that conclusively showed that Alvarez touched the ball twice, told a press conference.
"Did you see him touching the ball twice? Please whoever was present in the stadium and saw him touching the ball twice, the ball moving, please come forward and raise your hand!
"I don't see anybody with their hand raised so that's all I have to say... next question."
Simeone gestures to the Atletico fans after defeat (AP)
On TNT Sport, former Real Madrid player and Champions League winner Steve McManaman said it summed up his former team in Europe's top competition.
"Sometimes the best team doesn't win. Sometimes you need a huge chunk of luck to go through. Real Madrid have had luck in spades tonight," he said.
" Julian Alvarez slipping, an irregular kick disallowed. Antonio Rudiger sneaking in the last kick. Just wow. They got away with it.
"You can see why Atletico Madrid feel like the underdogs. They have given their all and they are out."
The laws of the game, set by IFAB, are clear when it comes to penalties: "the kicker may not play the ball a second time".
But how could VAR be sure to disallow Alvarez's penalty when the two replays shown by broadcasters did not offer definitive proof that there was a double touch?
The Champions League ball does not have a built-in chip, unlike with the ball used by Uefa at the Euros or by Fifa at the World Cup to determine an impact.
However, the Champions League does operate semi-automated offside technology (SOAT), which uses 30 in-stadium cameras and AI to automatically determine the 'kick-point' for offside decisions.
The VAR may have used that technology, rather than the video replays, to have ruled that Alvarez had touched the ball twice.
