Loris Karius made a poor mistake on his second European appearance for Besiktas after joining the Turkish giants on loan from Liverpool.
Besiktas were beaten 2-0 away to Malmo in Group I on Thursday, with Karius at fault for the opening goal.
A cross from the right wing took a deflection that deceived Karius, who attempted to tip the ball over his own crossbar.
But the goalkeeper only managed to flap at the ball, which landed in his own net, with the goal credited as an own goal by Caner Erkin.
Karius, who made two awful errors in Liverpool's defeat to Real Madrid in the Champions League final, was beaten again in the second half.
Erkin was booked for fouling Andreas Vindheim in the penalty box, with Markus Rosenberg stepping up to convert from 12 yards.
Group I is wide open after two rounds of fixtures, with all four teams having taken three points.
Karius kept his first clean sheet for Besiktas at the weekend as Senol Gunes' side beat Kayserispor 2-0 at home.