I watched that goal.
The circumstance was unfortunate:
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Dempsey Marchisio Altidore Barzagli Buffon
He had Dempsey covered WHEN the ball's crossed but then the ball goes to the other player, and he HAD TO focus on him. Had Marchisio watch Dempsey right when the ball reached that other guy and Dempsey was starting his run, he would react slower because he had to turn his head around. And it all happened for what, like 2 seconds?
He didn't even lose Dempsey in the end, he had two players in front of him (including Marchisio) and the shot got through.
My point is there's no need to blame anyone, it was a good move from opposition all around. Dempsey has always a good thing when it comes to shooting.