STAND-IN goalkeeper David Ospina had a night to forget as Arsenal slumped to a 3-2 home defeat against Olympiakos to leave their Champions League hopes in jeopardy.
Gunners boss Arsene Wenger had warned his side they already faced a must-win tie following their opening Group F loss at Dinamo Zagreb. But his side went behind against the run of play on 32 minutes when a low shot by Felipe Pardo deflected off Alex Oxlade-Chamberlain.
Theo Walcott netted a quick equaliser but Arsenal went behind again when Ospina, chosen ahead of Petr Cech, dropped a corner over the line.
Alexis Sanchez levelled on 65 minutes, but Arsenal hit the self-destruct button once more as Olympiakos immediately scored through substitute Alfred Finnbogason to give the Greeks a first win in England at the 13th attempt. Arsenal are now bottom of the group, and face a daunting double header against leaders Bayern Munich.
The visitors took a shock lead on 33 minutes. A corner from the right was cut back to the edge of the penalty area, where Pardo drilled in a low shot, which took a massive deflection off the heel of Oxlade-Chamberlain and flew past Ospina.
Arsenal were level within three minutes. Sanchez got away down the left, and fed Walcott, who beat the offside trap before cutting a low shot goalwards, which squirmed under goalkeeper Jimenez Roberto into the far corner.
No sooner had Arsenal recovered, they hit the self-destruct button again. A corner from the right was floated in by Kostas Fortounis, which Ospina jumped to collect, but inexplicably patted the ball down behind the line and the goal was immediately awarded by the Dutch fifth official Danny Makkelie.
Arsenal got on the front foot in the second half and equalised on 65 minutes. Walcott floated the ball in from the right, and Sanchez guided a cushioned header into the far corner.
However, the Emirates was silenced within a minute as Olympiakos regained the lead.
Arsenal failed to clear from the edge of their own penalty area, and the ball was put back in by Pardo, with Finnbogason clipping it past Ospina.
Scorers: Arsenal - Walcott (35), Sanchez (65); Olympiakos - Pardo (32), Ospina (40og), Finnbogason (66)
Referee: B Nijhuis (Ned)