It didn't take long for VAR to become a talking point as the video refereeing system made its debut in the Champions League group phase.
Liverpool coach Juergen Klopp was left livid over a penalty awarded to Napoli that helped the Italian side beat the defending champions 2-0 on the tournament's opening night.
Dries Mertens converted the penalty eight minutes from the end of an entertaining match after a contentious foul by Andy Robertson on Jose Callejon.
Substitute Fernando Llorente then sealed it in stoppage time with his first goal for Napoli to gain a measure of revenge after playing for Tottenham who lost in last season's final to Liverpool.
Klopp thought Callejon exaggerated the contact after Robertson was adjudged to have tripped the Spaniard in the area.
"I'm pretty sure there are different views on that. When a player jumps before there is contact it is not a penalty," Klopp said.
Klopp then wondered aloud how the referees came to the decision.
"I'm not the best loser but I'm not a bad loser. It was a decision made by human beings. As long as human beings make the decisions there is potential for failure.
"The referees and the VAR were all German, and they have experience with the VAR in Germany," said Klopp, who is also German. "I'm sure they will find an explanation on why they did the right thing."
Still, Klopp was not altogether discouraged.
"That was the game changer tonight and we have to accept the result," he said. "We are really critical with ourselves but it was not a really, really bad performance."
Liverpool became the first defending champions to lose their opening match since AC Milan were beaten by Ajax in 1994.
"It was very balanced match," Napoli coach Carlo Ancelotti said. "They're the best squad in Europe and they were again tonight, because they forced us to be extraordinary."
Last season, Napoli also beat Liverpool 1-0 at home before the English side won by the same score in England - denying Napoli a place in the knockout rounds.
"We put in practically the perfect performance," Callejon said. "We've faced them so many times that we know them tactically by now and did everything right."
Ancelotti, who added another chapter in his long, personal history against Liverpool, consoled Klopp.
"I told him to relax, because if he loses here, he tends to go on and win the Champions League," Ancelotti said. "I reassured him."