Video assistant referees (VARs) have been trialled in recent international friendlies and are currently being used at the Confederations Cup in Russia.
Portugal had an early effort controversially disallowed by the video referees during their 2-2 draw with Mexico on Sunday afternoon- according to rtesoccer
Defender Pepe thought he had opened the scoring in the 21st minute when he turned in a cross-shot after Cristiano Ronaldo had smashed a shot against the crossbar, having blasted his initial free-kick into the defensive wall.
However, Argentinian referee Nestor Pitana called for a review from the VAR and it was eventually ruled that Pepe was one of four players who were offside from Joao Moutinho's first cross into the box following the failed set-piece.
There was more controversy in the evening kick-off between Chile and Cameroon.
Eduardo Vargas thought he'd put the Chileans ahead in the first half but, after some deliberation, the VAR judged he'd been in an offside position when he raced on to a through ball and slotted home.
Chile coach Juan Antonio Pizzi believes more time is needed for new technology to be understood after his team felt the positive and negative results of the Video Assistant Referee (VAR) system being tested at the Confederations Cup in Russia.
“I think this system needs time. If I understand it correctly, we are trialling the system right now and there are maybe doubts because we are used to other situations in football," said Pizzi.
“I think we have to wait and see and maybe as we start getting use to it, it will get better,” he told a news conference after the opening Group B game.
“Having our goal at the end of the first half disallowed created stress for the players because they were not used to such a situation. But, on the otter hand, technology in the end will probably mean more justice in the game.
“The only negative about today’s incident with the VAR was that it is difficult for the players focusing in the game to absorb, and even if it is the right decision it has an impact on their psyche.”
Arturo Vidal, who had protested to the match officials, had this to say after the match "It's the first time we have played with this new technology and it's true while out on the pitch you are concentrating so that when it happens against you, it's difficult to comprehend.”
Under the VAR system, the referee can communicate with two video assistant referees sitting in a room in the stadium with access to all camera channels.
They look at goals scored, penalties awarded, straight red cards and cases of mistaken identity, when a player given a yellow or red card is not actually involved in an incident.
The VARs are allowed to correct the referee while the match official can refer decisions to them for a review.
Football's law-making body IFAB is expected to decide in March whether to authorise the use of VARs on a permanent basis.