Arsenal threw away a two-goal lead to draw 2-2 at Schalke and leave qualification from Champions League Group B wide open.
The Gunners overcame a slow start to go 2-0 up through Theo Walcott and Olivier Giroud only for Klaas-Jan Huntelaar to pull one back on the stroke of half-time.
Jefferson Farfan fired in at the far post in the second half to level and Arsenal held on for the point.
With Olympiacos winning 3-1 at home to Montpellier, Schalke stay top of the group with eight points, Arsenal have seven and the Greek side six. Montpellier have one.
Walcott, a doubt with a stomach bug, started on the right with Lukas Podolski on the left and Giroud at the point of an attacking line-up which sought revenge for the 2-0 defeat in the reverse fixture at the Emirates two weeks ago.
Schalke left-back Christian Fuchs had a shot deflected wide before former Barcelona winger Ibrahim Afellay forced the first save out of Vito Mannone on nine minutes, dribbling over halfway on the counter and letting fly with a low shot to the near post from just outside the box.
Home right-back Atsuto Uchida - withdrawn through injury later in the first half - had a glorious opportunity to pull back for one of his attackers from the right byline but allowed the ball to drift out of play while looking up.
And four minutes later, on 18, Arsenal scored against the run of play. Roman Neustadter misjudged a header in midfield to allow Giroud through on goal. He dallied long enough to be tackled, but Walcott was alongside to flick the ball above the diving Lars Unnerstall and tap in.
Eight minutes later it was 2-0. Giroud fought for the ball on the edge of the box and managed to get it out to Podolski, who whipped in a cross from the left for the Frenchman to finish at the near post with his head.
Schalke were shell-shocked and struggled to create in the final third despite seeing plenty of the ball, with the Gunners' defence solid.
Peruvian Farfan went closest to scoring for them with a long-distance drive that flew just wide before Huntelaar picked out the bottom corner with seconds remaining in the half.
It was poor play from Santi Cazorla that led to the chance, losing the ball in midfield. Lewis Holtby, a reported target for Arsenal, played a visionary one-touch pass to set up the prolific marksman.
Moments into the second period Giroud volleyed a golden chance into the ground and over from close range following excellent work by Podolski while at the other end Huntelaar found himself utterly alone in front of goal but his low shot was saved by the legs of Mannone, who was excellent on the night. In the former Real Madrid striker's defence, the chance was unexpected following a poorly-judged header from Per Mertesacker.
Schalke's vibrant fans inside Gelsenkirchen's Veltins-Arena screamed for a penalty when Benedikt Hoewedes's shot was blocked by the German defender's arm, but Italian referee Nicola Rizzoli waved away the appeals.
The pressure continued when on the hour mark Marco Hoger hit a shot on the turn that seemed to come off the outside of the post, but in fact was chested behind by Mikel Arteta, employed defensively by Arsene Wenger.
Holtby forced another save out of Mannone with a viciously swerving shot before the equalising goal on 67 minutes. Holtby arrived in the box but failed to get a clean head on the ball; Farfan was on the far side of the box though to slam inside the near post via the boot of Thomas Vermaelen.
Two minutes later Holtby would have picked out Huntelaar in the middle for a clear-cut chance but for a bobble as he crossed, while Arsenal were reduced to keeping the Bundesliga side at bay for the remainder of the match - until Walcott had a great chance in the fourth and final minute of injury time.
The England star went through on the left side of centre and looked certain to finish but Unnerstall stood tall just inside his box to block the low, weak shot.
Schalke deserved the three points, but will be content to have come from behind to remain atop the group while Wenger saw a better performance from his men than they put in for the home loss to Schalke, as well as in defeat at Old Trafford at the weekend. However there is plenty of room for improvement.
MAN OF THE MATCH
Vito Mannone - The oft-maligned keeper kept Arsenal in the game in the second half with some excellent saves.
SCHALKE: Unnerstall 6, Howedes 7, Uchida 5, Fuchs 7, Matip 6, Holtby 8, Jones 6, Neustadter 6, Afellay 7, Farfan 7, Huntelaar 7. Subs: Papadopoulos 6, Hoger 7, Barnetta 6
ARSENAL: Mannone 8, Sagna 6, Mertesacker 6, Vermaelen 6, Koscielny 7, Arteta 7, Wilshere 6, Walcott 7, Cazorla 5, Giroud 7, Podolski 7. Subs: Santos 6, Coquelin 6