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Before you get on me for this one, I'll clarify that "bad" is harsh for this discussion. Seeing as Arsenal were pretty impressive today, it might be better to term this "less than perfect."
This one goes to the defense, who were lucky to avoid a few penalties.
Now, I'm not going to go so far as to say there were missed calls that influenced the result, but there were clearly some moments where Arsenal were vulnerable.
Look at Per Mertesacker's pull-down of Luis Suarez in the box. Another referee could easily have whistled that, while a different striker could have stayed on his feet and put the ball in. Mertesacker should have positioned himself better.
In another instance, Thomas Vermaelen fell on the ball and allowed it to touch his arm. A penalty undoubtedly would have been harsh in that instance, but Vermaelen opened himself up to the discretion of the referee with his lack of strength in that play.
Of course, the result of the match is that Arsenal are still the only side yet to be beaten, which is absolutely fantastic.
However, they showed a bit of vulnerability that could be exploited by the likes of Chelsea or the Manchesters should they play similarly.