Vincent Aboubakar's header four minutes from time earned Besiktas victory at Napoli as the Turkish side edged a Champions League thriller 3-2.
The home side twice pegged back their visitors and thought they had gone ahead late on, only to see Manolo Gabbiadini's strike ruled out for offside.
And Aboubakar then nodded in his second goal of the game from Ricardo Quaresma's free kick late on, ensuring Napoli dropped points for the first time in this year's competition as Group B was blown wide open.
It was a third straight loss for Maurizio Sarri's side and they fell behind as early as the 12th minute.
Tolgay Arslan's flick around the corner was picked up by Quaersma and his drilled cross was converted at the back post by ex-Barcelona full-back Adriano, who had stolen a march on Jose Callejon.
Napoli levelled on the half-hour mark as Callejon made amends, collecting Marek Hamsik's long pass before leaving Dries Mertens with a simple finish in the six-yard box.
They were on level terms for just eight minutes as Jorginho's terrible back pass was latched onto by Aboubakar and he took one touch before sliding past Pepe Reina, though the goalkeeper will have been disappointed he could only help the ball on its way via his foot.
Five minutes into the second period the home side had a penalty when Fabri tripped Mertens having tried to reach the ball after saving from Callejon.
However, the spot-kick from Lorenzo Insigne, which came at the end of a laboured run-up, was easily saved by the Besiktas stopper.
He could not save a second penalty, though, as Mertens won another set-piece inside the area after Caner Erkin put his arms around him, and this time substitute Gabbiadini stroked home with his first touch.
Gabbiadini then thought he had put Napoli ahead with a close-range bicycle kick after Marcelo diverted Callejon's pass against his crossbar, but the assistant referee's flag was raised for offside.
And an action-packed game concluded with Aboubakar's winner from Quarersma's delivery, the third Besiktas goal from their third shot on target, with the hosts appealing for an offside flag that never came.
It means Napoli, Besiktas and Benfica, who won against Dynamo Kiev, are now separated by just two points at the group's halfway stage.