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No. 25 USC
Yes, the Trojans won, but barely. To make matters worse, the team they almost lost to was Minnesota. Not exactly a top level competitor.
USC was already barely in the rankings and there were enough impressive performances by other teams to knock them off for now.
No. 23 Auburn
Wow. Auburn barely escaped the Utah State game with a W. Utah State only won four games last year and that was playing in the WAC.
Auburn was great last year, but they lost too much to stay good. They had a great recruiting class though, so maybe they'll be good again in 2012 or 2013, but for now, they're not a top 25 team.
No. 16 Notre Dame
This year, Notre Dame was for real. They made it up to No. 16 in the AP polls based on hype and tradition, but neither of those things help you on the field, as South Florida showed them.
While it was a pretty close game, you don't host a mid-level Big East team and lose if you're the 16th best team in the country.