Gunners Manager Arsène Wenger expressed his frustration with his side conceding a late goal against Dutch team AZ Alkmaar.
Arsenal were leading 1-0 for most of the match and could have killed off the game on several occasions if not for woeful finishing.
Wenger, who turns 60 on Thursday, also admitted that his side were still learning from their Champions League experiences.
However, the Gunners remain in a strong position in Group H after three games, and are still unbeaten.
Wenger said: “In the Champions League you need focus for 93 minutes – we made one mistake and paid for it.
“You know that in the last five minutes of a Champions League match, anything can happen.
“They have thrown everything forward and I felt they could only score on a set-piece – and that is what happened.
“It is frustrating because we looked closer to scoring a second than conceding, but credit to them, they did fight well and went until the last minute.
“You do not need to give a free-kick away on an offside in the last minute.
“We were largely in control, but as long as you are not two goals in front, that is the problem.
“We are still learning and are still in a strong position. We have three games left, two at home, and I believe to win one game will qualify us.
“What I want is for us to continue to play like we do, focus to play better. In football, you have to produce the maximum result with your game, and tonight that did not happen.”