Sunderland 2-0 Arsenal
Arsenal's troubled season took another turn for the worse after they were deservedly beaten by a rejuvenated Sunderland in the FA Cup fifth round.
Fourth in the Premier League and facing likely elimination in the Champions League, the Gunners are set for a seventh successive trophyless year.
Kieran Richardson put Sunderland ahead with a deflected shot before half-time.
Arsenal pushed for an equaliser but Sebastian Larsson's shot was put into his own net by Alex Oxlade-Chamberlain.
It was an unfortunate moment for the 18-year-old, who was one of Arsenal's more impressive performers on a dark day for the club.
Arsene Wenger will inevitably come under increased pressure but not for the first time this week he was badly let down by his players.
Wenger demanded a response from them following - but he got nothing of the sort.
The 10-time winners were out-thought, out-fought and out-worked by two-time winners Sunderland, who avenged their a week previously and have won 10 of their 15 games under Martin O'Neill.
Wenger's current team clearly lack the quality of previous squads and they will not be relishing next Sunday's Premier League visit of high-flying local rivals Tottenham Hotspur.
Arsenal actually looked sprightly in the opening minutes, passing and moving with an urgency that showed their determination to prove a point following the humbling in Italy.
Gervinho, back from the Africa Cup of Nations and one of five Arsenal changes, showed what the Gunners had been missing in his six-week absence with a darting run down the left.
Mikel Arteta then curled a free-kick narrowly wide as the visitors put Sunderland on the back foot and probed for an opener.
But, all season long, Arsenal have reacted poorly to even the smallest of setbacks, and so it proved again as left-back Francis Coquelin went off with a hamstring injury in the 10th minute.
He was replaced by Sebastien Squillaci, who partnered Djourou in central defence, with Thomas Vermaelen shifting across, and, from that moment on, Arsenal's back line appeared vulnerable.
They received little protection from a midfield in which Aaron Ramsey and Alex Song took knocks, but that could not excuse individual errors and a general lack of concentration and organisation.
Song was fortunate to escape after the ball struck his hand inside the penalty area before Djourou lost possession on the left and earned a caution for hauling down Craig Gardner.
The resultant free-kick from Larsson was headed out to Richardson on the left and, under minimal pressure, his drive deflected off Squillaci and into the bottom corner.
Before that, Gervinho drew a fine save from Simon Mignolet and Arsenal wanted a penalty for what looked like a perfectly-timed challenge on Robin van Persie by John O'Shea.
But those were their only clear openings in a half that saw the Gunners become increasingly ragged as Sunderland denied them time and space with an intelligent and energetic display.
The tone was set by tireless captain Lee Cattermole, but it would be unfair to highlight individuals in a team whose hunger and cohesion could not be matched by their opponents.
It was a shame that only 26,042 spectators - just over half-capacity - were at the Stadium of Light to witness their efforts.
Sunderland came close to a second before half-time - James McClean firing into the side-netting after Jack Colback's knockdown from another Larsson cross - but Arsenal held on.
The 4,000 away supporters urged Arsenal forward after the break but there was no structure to their attacks and Sunderland comfortably repelled everything thrown at them.
Wenger made a double substitution on 52 minutes - Theo Walcott and Tomas Rosicky coming on for Aaron Ramsey and the apparently injured Squillaci - but the difference was barely noticeable.
The killer blow arrived when Stephane Sessegnon robbed Oxlade-Chamberlain of possession, surged at the bedraggled Arsenal rearguard and fed Larsson on the right.
The ex-Gunner's shot came back off the post but Oxlade-Chamberlain inadvertently bundled it over the line. It summed up Arsenal's day and, perhaps, their entire campaign.