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Taoufik Makhoufi was in first place coming down the home stretch. As he threw his arms out in celebration, he got passed by Robert Biwott of Kenya right before the finish line.
Here's the end of the race. You can see Makhoufi put his arms to his sides like an airplane (via Deadspin):
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The full celebration. His face slackens when he realizes what happened:
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Nope:
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He lost by 0.04 seconds:
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Here's video of the race. The announcer can't believe it: