Arsenal's Champions League hopes were left dangling by a thread on Tuesday after they lost an astonishing clash 3-2 to Olympiakos Piraeus who won a competitive European match in England for the first time in 50 years.
After losing their opening game to Dinamo Zagreb two weeks ago, Arsenal are bottom of Group F, six points behind leaders Bayern Munich, who they have to play twice.
Olympiakos, whose 12 previous visits to England ended in defeat, went ahead when Felipe Pardo's 32nd minute shot was deflected by Alex Oxlade-Chamberlain and flew past goalkeeper David Ospina.
Arsenal were level three minutes later when Theo Walcott scored but Olympiakos went in front again when Ospina mishandled a corner and dropped the ball over his line with the fifth official signalling a goal.
Alexis Sanchez equalised for the hosts with a neat header in the 65th but almost immediately Icelandic substitute Alfred Finnbogason put Olympiakos ahead again and they held on to seal a famous victory at The Emirates.
Arsenal manager Arsene Wenger was criticised for his team selection when they lost 2-1 to Dinamo Zagreb in their opening match and now faces more wrath from the fans for preferring Ospina to usual first-choice Petr Cech in goal.
Ospina made a calamitous mistake to gift Olympiakos a 2-1 lead after 40 minutes, failing to collect an inswinging corner when unchallenged and allowing the ball to drop over the line.
That was the third goal in an eight-minute spell after Pardo opened the scoring with a shot from the edge of the box which went in off Oxlade-Chamberlain's heel leaving Ospina helpless.
Arsenal equalised three minutes later when Walcott netted for the 16th time in his last 20 starts. Alexis Sanchez created the chance but Roberto should have done better with Walcott's side-footed shot from a tight angle.
Arsenal looked far more dangerous after the break and almost equalised on the hour when first Per Mertesacker then Santi Cazorla thundered in shots which were saved by Roberto and then Omar Elabdellaoui on the line before Sanchez found the net.
They were level for less than a minute as halftime substitute Finnbogason finished off a neat passing move to stun the Arsenal faithful with a goal that proved to be the winner.