In the situation leading to De Bruyne’s equalizer against Real Madrid, the ball went all the way but the referee did not detect this error.
The ball went out of bounds before De Bruyne scored
In the 67th minute, Man City suddenly found an equalizer against Real Madrid in the context of many difficulties. De Bruyne’s flash of a shot from outside the box brought the game back to the starting line.
It is worth mentioning that this goal does not seem to be valid. About 20 seconds earlier, Bernardo Silva had saved the ball close to the touchline after Walker’s cross. Observing this situation, some Real Madrid players and coach Ancelotti thought that the ball went out of the field.

Because the ball is very fast, the referee cannot determine whether the ball has gone all the way to the goal line or not. VAR didn’t get involved either. Therefore, the game continues.
Thanks to that, Man City continued to attack and the ball was ended with De Bruyne scoring. After the game, beIN Sports used 3D technology to examine the controversial ball. The results showed that the ball completely went out of the touchline and Real Madrid conceded an unfair goal.
Coach Ancelotti was even given a yellow card for overreacting to the referee about this situation. The semi-final first leg of the Champions League between Real Madrid and Man City ended with a 1-1 draw. Before that, Vinicius opened the scoring for Los Blancos in the first half.
Source: https://bongda24h.vn/bong-da-chau-au/ban-thang-cua-de-bruyne-vao-luoi-real-khong-hop-le-184-352600.html