Olympiakos stunned Arsenal with a 3-2 win at the Emirates as the Gunners were consigned to a second successive Champions League defeat.
The Greek champions - who had lost their previous 12 games against English opposition away from home - ran out winners to ensure a worst ever start to the competition for Arsenal.
The loss, combined with the 2-1 defeat at Dinamo Zagreb a fortnight ago, means the Gunners are now rock bottom in Group A and with back-to-back games against Bayern Munich next on the European fixture list, things are looking bleak for the Premier League club.
This was truly a night to forget for Arsene Wenger, who had spoken of the match's 'must-win' status in its build-up only to watch his side go behind after 32 minutes to Felipe Pardo's deflected shot.
The goal was hardly against the run of play - Arsenal had looked dangerous, particularly on the counter-attack, but the contest had been an even one until that point and although the deflection was crucial this was not a freak goal in any way.
Theo Walcott equalised a little more than two minutes later when he converted Alexis Sanchez's pass but the hosts went into the interval behind after a bizarre own goal by goalkeeper David Ospina, who was controversially selected ahead of £11m summer signing Petr Cech.
Ospina made a horrible mess of collecting an Olympiakos corner, spilling the ball over his own goal line to restore the Greeks' advantage. The goal was awarded by the 'fifth official' behind the goal.
The decision to leave Cech on the bench is likely to lead to some strong criticism of Wenger, especially if Arsenal fail to qualify for the knockout stages of the tournament, especially as the former Chelsea man was the club's only major signing of the summer transfer window.
After going behind, the home side dominated possession but a combination of some slow build-up play and their opponents' resilience and organisation in defence made them a hard proposition to break down.
Repeatedly Arsenal got into good positions but did not have either the pace or the guile to puncture an obdurate and deep-lying Olympiakos defence which was bolstered at times by all 10 outfield players.
Walcott eventually returned the creative favour for Sanchez with a clipped cross for the Chilean to head home an equaliser on 65 minutes and that should have been the cue for Arsenal to climb through their attacking gears and overwhelm their opponents.
But less than a minute passed before Olympiakos were again in front, with former Leicester midfielder Esteban Cambiasso playing a clever pass in behind a messy backline for Pardo to run onto and pick out substitute striker Alfred Finnbogason, who tapped in with ease.
Arsenal again had the bulk of the ball but invention was in short supply; when they did breach the packed defence they found goalkeeper Roberton in inspired form.
He had already produced a fine triple-save to keep out efforts from Sanchez, Per Mertesacker and Santi Cazorla in quick succession when he added an excellent full-length stop to keep out a Mesut Ozil free-kick to his collection.
His performance paved the way for a famous Olympiakos victory and puts a major dent in the Gunners' Champions League hopes.