Arsenal supporters have hit out at their side's failed offside trap after Nottingham Forest's Eric Lichaj exploited space in behind to head home Kieran Dowell's free-kick.
The Gunners set up a relatively high-line as several blue shirts camped on the edge of the six-yard box, leaving a gaping space between themselves and goalkeeper David Ospina.
The tactic was to catch any Forest players wandering offside while Dowell sent in the free-kick, only for the Arsenal players to realise that their two-man wall was playing an unmarked Lichaj onside to break the deadlock.
The goal was the first of four for the Championship side who went on to claim a famous third-round FA Cup victory over Arsene Wenger's side.