Vinicius Junior scored and then alleged he had been racially abused during Real Madrid's 1-0 win away to Benfica in the first leg of their Champions League play-off tie on Tuesday, while Paris Saint-Germain came from two goals down to beat Monaco 3-2.
Elsewhere, Galatasaray stunned Juventus, beating the Italians 5-2 in Istanbul, and Borussia Dortmund seized the upper hand in their tie against Atalanta with a 2-0 first-leg victory.
Madrid headed to Lisbon to play Benfica less than three weeks after a dramatic 4-2 defeat away to the same opponents, when goalkeeper Anatoliy Trubin scored a stunning late goal to take the Portuguese side through to the knock-out stages and force Real to drop into the play-offs.
Benfica coach Jose Mourinho was hoping to again inspire his current side to victory against the club he coached from 2010 to 2013, but the night was instead overshadowed by the alleged racist abuse reported by Vinicius.
The Brazilian scored a superb goal early in the second half, arrowing his shot into the top corner. His moment of brilliance, though, quickly became controversial when he was booked for his dancing celebration in front of the crowd.
That booking sparked an exchange with Benfica's Gianluca Prestianni, who pulled his shirt over his mouth before appearing to say something to Vinicius.
The Real forward then ran over to referee Francois Letexier and, pointing at Prestianni, seemed to claim he had been called "mono", the Spanish word for monkey.
That led to a hold-up of several minutes before the game resumed, with Vinicius then being jeered by the home fans.
Mourinho was then sent off in the final stages of a spiky game for complaining too vociferously as he appealed for a second yellow card for Vinicius, meaning he will not be on the touchline for the second leg at the Bernabeu next week.
The long stoppage led to 12 minutes of added-on time, but Benfica could not use them to find an equaliser, and face an uphill struggle in the Spanish capital next week.
"(What happened) is something that nobody likes, you have to ask the Benfica player what he said," Real coach Alvaro Arbeloa told broadcaster Movistar.
"(Racism) is something we have to eradicate from the world of football and if the players don't fix it then this is very complicated."
GALATASARAY STUN JUVENTUS
In Monaco, reigning European champions PSG recovered from a shambolic start as Desire Doue inspired a comeback from two goals down to beat the principality side.
United States star Folarin Balogun opened the scoring after just 55 seconds and found the net again on 18 minutes to put Monaco in control.
Vitinha then missed a chance for PSG to reduce the deficit when he had a penalty saved and last year's Ballon d'Or winner Ousmane Dembele came off injured before the half-hour mark, but his replacement Doue was the hero.
He pulled one back within two minutes of coming on, and then helped set up Achraf Hakimi for the equaliser just before the break.
Monaco then had Aleksandr Golovin sent off right at the start of the second half for a challenge on Vitinha before Doue got the winner on 67 minutes.
"He is an incredible player and a different player. I am very pleased for him because he deserved it," PSG coach Luis Enrique told broadcaster Canal Plus of Doue.
Earlier Galatasaray produced a rousing performance to beat Juventus 5-2 in a pulsating contest in Turkey, with Noa Lang scoring twice for the hosts.
Gabriel Sara put the home team in front but Teun Koopmeiners scored twice to put Juventus ahead at the break.
However, Lang made it 2-2 just after halftime and Davinson Sanchez put Galatasaray back in the lead before Juventus had Juan Cabal sent off.
Lang got his second and substitute Sacha Boey sealed the win to leave two-time champions Juventus facing an uphill struggle in next week's return.
Dortmund, beaten finalists in 2024, are on course to eliminate another Italian team after beating Atalanta 2-0 with Serhou Guirassy opening the scoring early on.
His 16th goal of the season was followed by a Maximilian Beier strike just before the break.