The Gunners dropped what could be two huge points in the title race when Ivan Toney headed in after a scramble inside the Arsenal area.
A lengthy VAR check took place after the goal to examine whether Ethan Pinnock was offside and interfering with play earlier in the move.
But VAR officials then missed the fact Christian Norgaard was in an offside position when he hooked the ball across for Toney to equalise.
Brentford's goal was not the only controversy of a bad Saturday for officiating in the Premier League.
Brighton had a goal wrongly ruled out in their 1-1 draw with Crystal Palace.
Pervis Estupinan's strike was disallowed after a VAR review deemed the Ecuadorian to be offside.
However, the lines drawn by the VAR official to determine the offside used the wrong Palace defender with Marc Guehi stood behind James Tomkins.
"PGMOL can confirm its Chief Refereeing Officer Howard Webb has contacted both Arsenal and Brighton & Hove Albion to acknowledge and explain the significant errors in the VAR process in their respective Premier League fixtures on Saturday," said a statement issued by PGMOL.
"Both incidents, which were due to human error and related to the analysis of offside situations, are being thoroughly reviewed by PGMOL."
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