Imagine this on Saturday morning (Singapore time). It's the 89th minute of the World Cup Group B clash between Portugal and Spain at the Fisht Stadium in Sochi, Russia.

Cristiano Ronaldo is felled in the penalty area by David de Gea. Or has he? It all happened so quickly. The referee gulps nervously and traces a box in the air with his fingers. This means VAR (video assistant referee). And VAR could mean chaos.

This summer, for the first time in the World Cup's 88-year history, referees will be able to call upon video assistance for contentious decisions.

Hypothetically, the move should help eliminate mistakes from the game. But VAR is not foolproof and early experiments in some of the world's major leagues have created growing anxiety at what might come to pass in Russia.

Could the biggest tournament in world football be ruined because of computer glitches?

Take the aforementioned hypothetical example of Ronaldo tumbling over de Gea.

Can video replays ever provide conclusive evidence of a contact foul? If they can, then why do pundits and fans around the world continue to debate decisions, even after reviewing them dozens of times from multiple angles?

Even with a clear line of sight, it can be impossible to ascertain the truth.

Fifa president Gianni Infantino has argued that the new technology will have a positive impact on football.

"From almost 1,000 live matches that were part of the experiment, the level of accuracy increased from 93 per cent to 99 per cent," he said.

And yet there were moments this season when VAR failed to convince.

In February, Manchester United were denied a goal by VAR for an alleged offside, "proved" by a wobbly yellow line that seemed to have been drawn around Juan Mata.

In Germany, a referee was forced to recall Mainz and Freiburg players from the dressing room at half-time to take a penalty that hadn't been noticed when they were on the pitch.

In Australia, a "malfunction of software", meant that a crucial and evidently offside goal in the A-League grand final was missed, although in this case the human officials missed it too.

And while it's one thing for the television audience to enjoy the mounting tension created by VAR, the reality in the stadium is very different.

Without the communication enjoyed by sports like American football, supporters can be left baffled by unexplained events on the pitch.

There was widespread derision from the Wembley crowd in February when Tottenham Hotspur's FA Cup replay with Rochdale was repeatedly delayed by consultation between the referee and his team.

And this was a one-sided game with relatively little pressure. Imagine how the crowd might react in a tense knockout game.

Fifa has promised to develop a VAR information system to ensure that "a Fifa staff member informs the broadcasters, commentators and infotainment about the different steps of the review process", which include "VAR-specific graphic templates for TV and the giant screen in the stadium".

But, in the cauldron of an excitable football stadium, that may not be enough.

Spurs manager Mauricio Pochettino, speaking after that odd night at Wembley, is one of many who believe that the referees, and their human weaknesses, are just part of the game.

"My opinion is that we have the best referees in Europe," he said. "The referee is the boss on the pitch and has the last word always."

Not now, he doesn't. And all of football will have to hold their breath and hope that VAR doesn't go horribly wrong in Russia.

• Iain Macintosh is a football writer and broadcaster. Formerly with ESPNFC, he can be heard on the The Totally Football Show podcast, available on iTunes.