The Arsenal centre-back was shown a straight red card for pulling down Edin Dzeko in the box but did Mike Dean make the correct call in giving the defender his marching orders?
Koscielny appeared to haul the former Wolfsburg striker to the ground as he was about to shoot from close range and to the bemusement of Gunners fans and players alike referee Mike Dean pointed to the spot.
A red card for the Arsenal centre-back duly followed with the match official deeming that Koscielny had denied Dzeko a clear goalscoring opportunity.
The drama wasn't over there, however, as Dzeko dusted himself off and took the resulting spot-kick only for Wojciech Szczesny to divert the Bosnia & Herzegovina attacker's effort onto the post and across the line before he was safely able to gather and avert the danger.
Let us know what YOU think about what could prove a key incident in deciding the game at the Emirates Stadium. Did Koscielny deserve to be sent off or has referee Dean made a mistake? Let us know in the comments section below and be sure to vote in our Facebook poll!