Arsenal had taken the lead midway through the first half thanks to Alexandre Lacazette.
Unai Emery’s men were seeking a ninth straight win in all competitions.
The Gunners have looked reasonably solid at the back in recent games.
But they were undone after an error at the back moments before the break at Craven Cottage.
And Arsenal fans laid the blame at Nacho Monreal for gifting Fulham the chance to attack.
As these images show, Monreal takes the ball out of defence but then loses it when playing forward.
And within seconds the Fulham midfield have moved the ball through to Andre Schurrle, who clipped home for 1-1.
“Thing is with Monreal is that this season he has been pretty good going forward (see goal) but horrendous at passing at defensively. We really need a new left back,” one fan wrote on twitter.
“Nacho Monreal. From hero to zero,” another said.
A third fan put: “Bellerin and Xhaka need to be dropped. Monreal is lacking”
A fourth simply said: “Blunder from Monreal”
And Michael Owen was critical of the defender at half time.
“Monreal, in that situation, you just simply can’t give the ball away,” Owen said on BT Sport.
“If you’re out of position yourself, then you’ve just got to go long.
“Even if you give the ball away but it’s long, at least it gives you time to get back in position.
“But hitting the ball short and being totally out of the game is just a no no.”