Ivan Toney's equalizing goal sparked controversy.
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The English Premier League has long been decided by slim margins: a point lost here, a goal missed there.
Just one point separated last year's champions Manchester City from runners-up Liverpool and, as Jürgen Klopp's mid-table side are now learning, the title chances don't present themselves. often.
So when "human error", acknowledged by the body responsible for Premier League referees, allowed Brentford's equalizing goal to remain on the scoreboard (1-1) against league leaders Arsenal , provoked an angry reaction, as it deprived the Gunners of a crucial victory.
"I just looked up and it's offside, yes," coach Mikel Arteta told Sky Sports.
"Did they explain to me why it wasn't allowed? No. It's frustrating, but we'll probably get an explanation later in the week. We don't have one today."
With Brentford trailing 1-0, Ivan Toney scored with a header after the ball drifted into the box from a free-kick.
Video assistant referee (VAR) Lee Mason checked Ethan Pinnock's tackle on Gabriel during the kick-off, but appeared not to use VAR guidelines to check a second incident on goal play, when Christian Norgaard appeared to be in offside.
Arsenal is the leader of the Premier League.
(Credit: David Price/Arsenal FC/Getty Images)
"You have to apply certain principles when defending and that is done by respecting the rules. Suddenly you change the rules and then you have to change your principles," continued Arteta.
"So tell us beforehand so you don't keep the (defensive) line so high later. Because you're always going to have an advantage if you get blocked. You can't block if you're offside."
Professional Game Match Officials Limited (PGMOL), the body responsible for the Premier League's referees, told CNN that its head of refereeing Howard Webb had contacted Arsenal "to acknowledge and explain significant errors in the match process. VAR".
It added that the bug was "being thoroughly reviewed by PGMOL."
Aaron Ramsdale, Arsenal goalkeeper, is kicked by a fan after a derby against Tottenham
Arsenal's draw against Brentford marked the second week in a row in which they have dropped points after the shock defeat to Everton, and their lead over Manchester City is now just six points, instead of eight if they had won against him. Brentford.
The Gunners will be back on the pitch on Wednesday, against City, in a clash that is likely to define their season.
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