VAR technology is back in the spotlight after Manchester United's Juan Mata was controversially denied a goal in a confusing incident that raises questions over whether it is ready for a Premier League rollout.
The video assistant referee system has been trialled in hundreds of matches around the world to review key moments linked to goals, penalty decisions, red-card incidents and cases of mistaken identity.
Football's governing body FIFA will decide next month whether to officially endorse the VAR system, a move that could see the controversial technology used at the World Cup.
The Premier League is lagging behind the Bundesliga and Serie A, which are testing the technology this season but Saturday's incident, which saw a Mata goal cancelled out after a lengthy delay, was just the latest in a series of contentious moments involving VAR in England.
AFP Sports looks at the key issues that must be resolved.
Is it reliable?
Hawk-Eye, whose goal-line technology used in the Premier League has been widely regarded as a success, apologised for providing "an incorrect graphic" to broadcasters during United's FA Cup fifth-round game at Huddersfield on Saturday.
"To confirm, the VAR saw the correct image with the correct lines to make the decision. This was a case of the wrong image being provided to the broadcaster and we apologise," Hawk-Eye said in a statement.
However, it raises doubts over a core concept -- will the technology be in a better position to make correct decisions than the match officials?
"Saturday made me wonder how far we can trust the technology to arrive at incontestable decisions, which is strange given that is the point of it," the former head of Premier League referees, Keith Hackett, wrote in the Telegraph newspaper.
United boss Jose Mourinho says there must be irrefutable proof that VAR can consistently lead to better decisions.
"I think our game is so beautiful that to change we have to change for better," said Mourinho, whose side won 2-0 despite the disallowed goal.
"The technology on the goal line I'm really, really happy with that. If they find a way in the VAR that the VAR is always right and always brings the truth, I'm happy with that."
Mata's disallowed goal also raised another criticism of the system -- the time it takes for a final decision to be made.
"I think in the future the VAR will be helpful for the referees, who always have a difficult job, and eventually it will bring more justice to football," Mata wrote in his online blog.
"Technology can help football to be more fair, especially nowadays when it seems that the result is the only thing that matters," he added.
"I believe, though, that such technology should be restricted to a few particular actions that are crucial in the game... and especially that the final decision should take less time and be more clear than last Saturday's."
West Brom coach Alan Pardew complained that two of his players suffered hamstring injuries due to delays in play as three major decisions were referred to VAR in his side's 3-2 win at Liverpool in the previous round of the FA Cup.
Fans in the dark
While replays of referred decisions are offered to television viewers, unlike in tennis, for example, fans inside the ground are not shown the pictures.
"In the stadium, it was bizarre, because we got no evidence of what happened," Pardew said about his experience at Anfield last month.
Despite the Mata decision going in his side's favour, Huddersfield boss David Wagner also expressed his displeasure at what he sees as robbing fans and players of the emotion of scoring a goal.
"Yes, the decision went in our favour but this VAR for me kills the emotion of the game," said Wagner. "This is why I don't like it."
© 2018 AFP