The Senegal worldwide tapped home after Roberto Firmino had hit the post but his effort was chalked off even though he was behind the ball, having initially been offside when the move started. Good saves to deny deny Virgil Van Dijk in either half but also misread a cross which allowed the defender to head on to the post, and should have done better in controlling the cross that led to the goal.
Alisson Becker spread himself well to block from Lacazette before Pierre-Emerick Aubameyang's deflected effort from the edge of the area flew just wide.
Leno partly atoned for that error by pushing over Van Dijk's 74th-minute header, by which stage Arsenal's earlier poise and threat looked to have dissipated.
When it came, in the 83rd minute through Alexandre Lacazette's curling strike, it was a moment that owed as much to the belief of the fans as the drive of the players.
What does it mean?
A rare win over a big six rival may have eluded Arsenal against Liverpool on Saturday but they showed signs of encouraging development under Unai Emery, writes Peter Smith. While his side performed well on one end of its League Cup victory over Derby County this week, he wasn't all too pleased with how they defended against the Championship side.
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Depending on the jurisdiction, high level sex offenders, who are out on parole can not participate in Halloween activities. Police have stepped up their monitoring of registered sex offenders to help keep trick-or-treaters safe this Halloween .
While Liverpool only took one point from fixtures against those big four teams, they now have five. They play the ball through and could score the goal. Xhaka's passing range was to the fore as well over the course of 90 minutes he appeared to enjoy, now freed from his recent toils as an emergency left-back. The same could not be said for Leno.
Liverpool nearly went in ahead at the interval as Leno followed the earlier example set by his counterpart when he charged from his line and got stuck in no-man's land, with Van Dijk flicking a header against the foot of the post.
Arsenal took on Liverpool last night in the Premier League. The Spaniard won the Europa League three times in succession with Sevilla, including one against Klopp's Liverpool in 2016, and is a formidable tactical mind.
Ramsey scored a sensational goal in Arsenal's 5-1 win over Fulham at Craven Cottage last month to open his account for the season. However, the Gunners have scored more goals after the break than any other side (19).
Melissa Reddy joined Dion and Paddy to discuss the evolution of Liverpool from daredevils of the Premier League to a more conservative, cautious version.