Lee Mason's refereeing was atrocious. Louis Saha was miles offside when Seamus Coleman launched his pass to him making him active. The linesman should have raised his flag there and then. End of story. However FIFA's rules are also clear when it comes to Koscielny's situation. An attacker will be penalized for playing the ball being in a previous offside position even if the shot or pass rebounds off an opponent and comes to him.
As many commentators have pointed out this rule prevents players from gaming the system by deliberately trying to get rebounds off goalkeepers or defenders to allow strikers to get onside. Even David Moyes stated that the goal should not have stood.
The aggrieved Koscielny had the last laugh as he headed Robin Van Persie's corner for a gritty comeback win. Another Everton lapse gifted an Andrey Arshavin's equalizer. Not the best display by the Gunners but they hung in tough.