It's two defeats in two games for Arsenal in this season's Champions League.
The Gunners slumped to a 3-2 at home to Olympiakos at the Emirates.
Arsene Wenger's side were not helped by a first-half clanger from goalkeeper David Ospina, who fumbled the ball over his own goal-line just before half-time to leave the scores at 2-1.
Theo Walcott had equalised five minutes before that after Felipe Pardo had put the Greek side in front.
Alexis Sanchez headed in an equaliser to Ospina's own goal with 25 minutes to go to seemingly put Arsenal back on a correct footing, but immediately after that Alfred Finnbogason converted what proved to be the winner.
Bayern Munich are comfortably top of Group F after an in-form Robert Lewandowski hat-trick helped them to a 5-0 victory at home to Dinamo Zagreb.
Chelsea also tasted defeat tonight going down 2-1 away to Porto. Andre Andre gave the home side the lead, before Willian equalised for the Londoners just before the half-time whistle from a free-kick.
However, defender Maicon headed in Porto's winner on 52 minutes.
Dynamo Kiev's 2-0 win away to Maccabi Tel Aviv sends them top of Group G.
Meanwhile, two goals in two minutes saw Barcelona come from behind to beat Bayer Leverkusen in Group E and there was something of a shock in Belarus, where BATE Borisov were 3-2 victors at home to Roma.
Zenit continue to top Group H following a 2-1 win at home to Gent.
While Valencia were 1-0 winners at Lyon.
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