There has been controversy almost every weekend since the use of video assistant referees was introduced to the Bundesliga. So what's the problem and why have similar systems been so successful in other sports?
Was Pierre-Emerick Aubameyang's goal in the 13th minute of Saturday's spectacular Revierderby against Schalke on Saturday actually a good goal? On-field official Denis Aytekin gave the goal and the video assistant referee (VAR) raised no objection. However, it was controversial. Some spectators saw the contact between Aubameyang's hand and the ball as being coincidental, while others saw it as deliberate effort to guide the ball into the net. It really wasn't clear cut, one way or the other. The Schalke fans, though, felt cheated.
The devil is in the implementation
Those who support the use of video assistant referees argue that their introduction is bound to make the game fairer. The problem is, how to implement it effectively.
"I am still a big supporter of the VAR. But only if it is used the way we were told it would be," Schalke sporting director Axel Schuster said.
However, in recent weeks there has been a lot of confusion. So why is this the case?
Under the guidelines set out by the International Football Association Board (IFAB), the VAR is only meant to come into play in four scenarios: on goals (whether a goal is legal or not), penalty decisions, red card decisions, and cases of mistaken identity in awarding a yellow or red card.
Just why the VARS raised their voices over all kinds of debatable calls in recent weeks seems to be a mystery to all involved. The reaction of the German Sport University Cologne to a request for comment on the issue shows just how sensitive a topic this currently is. The university is part of the Bundesliga's VAR project, in cooperation with the German football association (DFB) and the company that operates the league, the DFL. Nobody at the university was willing to speak about the VAR project at this time.
Successful use across the pond
In the United States, video review has long been used in several sports, but they go about it in a different way. In the National Football League, Major League Baseball or the National Hockey League, coaches can ask request that a play be reviewed by a video judge. The coaches of both teams involved in any given game have a limited number of "challenges" at their disposal.
IFAB, which is the body in charge of the laws of the game – and therefore responsible for implementing any changes – decided against allowing coaches to challenge plays in professional football.
International field hockey
Video review and opportunities to challenge questionable calls were introduced in international field hockey in 2008.
"It's not always without its problems, but the video review has been fine-tuned over the years," the coach of Germany's men's team, Stefan Kermas, said.
"Players can only call for a video review on plays involving a goal. It seems like things haven't been thoroughly thought through in football. In our sport the use of video reviews has led to more fairness," he added.
Should referees be video referees?
Schalke sporting director Schuster wonders whether on-field referees "who are accustomed to and are have been trained to make split-second decisions" are the right people for the job "when it's all about evaluating plays using a television screen."
"I would like to see a mixture of young referees and former players sitting in front of the monitors in Cologne," he said. This, he argued, would help to alleviate the problem of a lack of acceptance of the VAR system among referees.
A lot of work to do in other leagues too
The Bundesliga is not alone in having difficulties with the introduction of VAR. In Italy, Juventus' goalkeeping legend, Gianluigi Buffon has complained about the "frequent delays" since it was introduced in Italy.
"We have to ensure that it is used less frequently. You can't be going to the video replay on every questionable call," the head of Italy's referees, Nicola Rizzoli, said recently.
ThePrimera Liga can expect to have similar experiences when VAR is introduced in Spain next season.
Kermas, though, is confident that the use of VARs can be a success in the Bundesliga.
"You need to give the project more time, then it will have a positive effect on football," the German field hockey coach said.