With Paris Saint-Germain being held to 2-2 by Arsenal, Edinson Cavani broke free and had only goalkeeper David Ospina to beat and score his second of the night.
The PSG forward was far too nonchalant, shooting too early with a wayward long-range chip that was drifting off target before being easily gathered by Ospina.
After one glaring miss, Cavani still had time to make amends but flicked a header wide and the Champions League game finished 2-2 on Wednesday.
PSG seized control of Group A by moving ahead of Arsenal due to its superior head-to-head record after drawing 1-1 in Paris, but the lack of urgency illustrated by Cavani could prove costly in the round of 16.
The Qatari-owned club is chasing its first European title, and coach Unai Emery was satisfied for now.
“This was a test and they met my expectations with the performance,” Emery said.
Top spot secures a game in the round of 16 against a group runner-up but whether that is an advantage will become clearer only after the knockout-phase draw. German champion Bayern Munich and Manchester City will finish in second place and holder Real Madrid is also currently a runner-up.
“You cannot really guess if it’s good or bad (being second),” Arsenal manager Arsene Wenger said.
Group A is completely lopsided, with PSG and Arsenal locked on 11 points and assured of progressing, while Ludogorets and Basel, who drew 0-0, are both on two points.
This was Arsenal’s first real European test of the season at home, after putting six past Ludogorets and easily overcoming Basel 2-0.
Just like in Saturday’s 1-1 draw at Manchester United, the Gunners were inert initially against the French champions and fell behind in the 18th minute. Thiago Motta slipped the ball through to Blaise Matuidi, whose low cross was turned over the line by a sliding Cavani, who made it 16 goals in 16 games.
It was a fiery end to a tepid first half. First there was a lifeline for Arsenal in stoppage time: Mesut Ozil put the ball through to Alexis Sanchez and Grzegorz Krychowiak slid in, missing the ball and tripping the Chilean. From the penalty spot, Olivier Giroud finally made a contribution to the game by scoring.
Amid the celebrations, there was a feisty exchange between Aaron Ramsey and Cavani, who punched the Arsenal midfielder in the chest but escaped any sanction.
Arsenal was reinvigorated by the equalizer and, after the crossbar kept out Lucas Moura’s free kick in the 54th, took the lead in fortuitous circumstances.
Carl Jenkinson crossed from the right and the ball pinged around the penalty area as Ramsey’s shot was blocked by Marquinhos and his clearance rebounded off Marco Verratti into the net.
Arsenal returned the favor on the hour. Hatem Ben Arfa’s corner was met by Lucas’ header and Arsenal winger Alex Iwobi helped it over the line.
“My concern is that we were 2-1 up and allowed them to come back at us,” Wenger said. “It’s difficult to understand how easy we gave away a goal at a corner. We lack something in our creativity.”