Julian Alvarez slipped as he struck the penalty, but the ball flew into the top left corner.

It looked like it was 2-2 in the Champions League last 16 shootout between derby rivals Atletico and Real Madrid.

What happened next was another one of those fine margin VAR calls - but for a very unusual reason.

On review, Atletico's Alvarez was judged to have touched the ball twice.

That is against the rules so it was ruled out - with no option for a retake.

UK users can watch of video of the incident at the top of this page and see if they can spot the second touch.

Real then went 3-1 ahead before winning the shootout 4-2. All the talk afterward was about that decision.

Atletico midfielder Conor Gallagher's strike after 27 seconds had levelled the tie 2-2 on aggregate.

Neither side could then find the decisive goal in normal or extra time, leaving penalties to decide the outcome between the rivals for a sixth time in knockout games.

After the first three penalties had been successfully converted, former Manchester City forward Alvarez stepped up with the chance to make it 2-2.

The Argentina international slipped as he took his shot, but still managed to beat Thibaut Courtois.

Atletico fans were celebrating, but seconds before Fede Valverde fired home Real's next penalty, Polish referee Szymon Marciniak indicated Alvarez's spot-kick had been ruled out as the video assistant referee (VAR) got involved.

A review of the incident showed the forward touched the ball twice in one movement as he slipped while shooting.

The scoreboard in the ground displayed 2-2 after Alvarez's effort, no doubt leading to confusion in the stands, but at that stage Real actually led 3-1.

Although Atletico goalkeeper Oblak saved Lucas Vazquez's spot-kick to briefly give Atletico hope, Marcos Llorente hit the bar for the hosts and German Antonio Rudiger sent Real through in a tense finale.

Article 14.1 of the IFAB (International Football Association Board) laws of the game concerns penalties.

The rules state: "The kicker must not play the ball again until it has touched another player."

That is the same law that stops people scoring a rebound if their own penalty hits the post.

If the same happened from a penalty during regular play, an indirect free-kick would be awarded to the opposition.

Atletico coach Diego Simeone cast doubt on the decision, but hoped the officials had made the right call.

"The referee said when Julian got to the penalty spot he touched the ball with his standing foot, but the ball did not move," he said.

"I've never seen a penalty where they've called the VAR, but well, they would have seen that he touched it. I want to believe they saw he touched it.

"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."

But Real Madrid coach Carlo Ancelotti was satisfied the correct decision was made.

"They detected it. When we realised there was this doubt they had already detected it on VAR," he added.

"I saw it, I think he touches it with his left foot, a second touch."

Real Madrid goalkeeper Courtois accepted it was unfortunate for Atletico, but the rules meant it was rightly disallowed.

He told Uefa.com: "I felt that there was something weird going on, so we said it straight to the referee and then it was confirmed that there was a double touch and it didn't count, so that obviously gives us the edge.

"[Alvarez] slipped and he touched the ball twice and that is a missed penalty. You cannot touch the ball twice. That's bad luck but it's the rules."

Former Newcastle and Republic of Ireland goalkeeper Shay Given said on BBC Match of the Day: "Sometimes you see it happen where the ball is kicked off the other foot and then goes in a completely different direction.

"Here the ball doesn't really change direction but [Alvarez] does kick the ball off his [right] foot. The laws of the game are clear."

Yes – twice in the Premier League.

In 2017, Leicester's Riyad Mahrez had a penalty disallowed for a similar offence in his side's 2-1 loss to Manchester City, who he later joined.

The Algeria winger slipped as he took a penalty and the ball hit his standing foot before looping into the net.

"The shot was strange but the rule is clear. Two touches, the same as golf," said City manager Pep Guardiola at the time. "It is not normal."

And in January 2023, Fulham striker Aleksandar Mitrovic did the same thing, accidentally kicking the ball on to his standing foot as he slipped while striking it. The Cottagers lost the game 1-0 to his former club Newcastle.

But later that year, a penalty stood under similar circumstances in the Scottish Premiership.

Bojan Miovski seemed to slip as he took a late penalty for Aberdeen against St Mirren, with an apparent double touch taking it into the net in a 2-2 draw. A VAR review, though, allowed the goal to stand.

Spanish football expert Guillem Balague, who was at the Metropolitano Stadium, told BBC Sport:

"There was a typical exchange of strong words in the tunnel after the match, but nothing different to what happens after every match.

"It was a tense game - very much how Atletico wanted to play it - with the added bonus of having scored so early. The idea was to defend deep and counter-attack.

"Real Madrid won a penalty but Kylian Mbappe was not right. He struggled with his ankle and with a toenail that is coming off.

"You had the penalty missed by Vinicius Jr in normal time. Mbappe said to Vinicius, 'I don't feel like kicking this, I am not ready' and walked away from it, Vini took it and it went high.

"Then the penalty shootout. It was difficult for people to understand what went on. There were not instructions from VAR for people to understand what was going on.

"But, nobody complained and said this is not fair. Atletico goalkeeper Jan Oblak even said if I doubt the decisions then it is a disaster because you cannot trust anything then.

"So he touched it twice, the penalty was disallowed and Endrick had come on with the intention of taking the fifth penalty. But Carlo Ancelotti looked at Endrick and thought he wasn't ready so asked Antonio Rudiger to take it.

"Oblak told me he saw it late and couldn't stop it. He will try to forget what happened, but a very hard moment to take."

The win keeps Real Madrid's hopes of a record-extending 16th European Cup triumph alive, and maintains their impressive record against their city rivals: