Referee assistant, Nigel Bannister, advised referee Stuart Atwell to award a John Eustace own goal when it was quite clear that the ball was yards wide of the goal.
"It appears a mistake has been made," PGMOA assistant referee's manager Paul Rejer told Watford's official website.
"It will be reviewed by us. If it is a serious error, he will regret it.
"It was the assistant referee who gave the goal and from his position and angle it appeared the ball had crossed the line for a goal. He has made a human error.
"The referee has no way of knowing for definite; he was relying on his assistant referee to make that decision.
"Performances by referees and assistant referees are reviewed after every game, both on the day and by DVD."