Arsenal welcomed Everton to the Emirates Stadium on Friday evening for their Premier League clash.
The first half was a very poor affair with few chances for either side.
But the game was much more entertaining after the break.
Arsenal thought they had a penalty when Dani Ceballos was felled inside the box.
However, the penalty was ruled out after Nicolas Pepe was adjudged to have been offside by the smallest of margins.
The Gunners would have been reeling with that decision and they then fell behind 20 minutes later when Bernd Leno made a truly awful mistake.
Richarlison found himself in a threatening position but his attempt at a cross should easily have been gathered by Leno.
But the German stopper failed to collect and the ball hit his heel and rolled into the net.
Watch the moment below...
Oh no, Bernd. That is the definition of a shocker.
That mistake proved to be costly as Arsenal failed to find a equaliser and fell to a one goal defeat.
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Arsenal remain in ninth having amassed 46 points from 33 games. They could fall into the bottom half by the end of the weekend if results go against them.
Everton, meanwhile, are one place higher and six points better off than the Gunners.
Carlo Ancelotti's side are now just three points off the top four and are still well in the mix to qualify for the Champions League.