Oh, it won't happen.
It can't happen.
The voters and the fans and the networks would never, ever let it happen.
But how would you resolve the following?
Alabama momentum starts to wane in the third quarter, as AJ McCarron is knocked out of the game on a deliberate helmet-to-helmet hit (that is not called). Auburn rallies, taking the lead on a controversial play where the Tigers get away with something massively screwy, on the order of Missouri's Fifth Down, or scoring on a dive with 12 players on offense. Auburn wins close, a game that every rational observer will decry as a screw job. Something even more obvious than the way Wisconsin got hosed this season. A screw job so egregious, that it overcomes Bama Fatigue and Bama Schadenfreude.
Ohio State, having celebrated its ascension to the BCS top two, promptly derps a two touchdown defeat at the hands of Michigan State in the B1G Title Game. (Yes, it seems improbable that Sparty can put two touchdowns together. Maybe one is defense and one is special teams.)
Duke (or any of the other four contenders in that division) pulls the upset on Florida State in the ACCCG. Most likely to happen only if the Wheel of Destiny spins out a charge against Jameis Winston the night before the game, giving the Seminoles the least amount of time to give reps to a backup's backup.
Auburn beats Missouri in the SEC Title Game.
One more Oklahoma State loss, just for good measure. Lord knows they would whine about getting the short end of another Bama rematch.
One + Two + Three + Four + Five = BCS Armageddon
Never mind, the BCS is going away anyway. But given the above scenario, you'd have a top-ranked Auburn team, with a very worthy challenger just one spot behind -- one that objective observers would agree had been hosed on the road. And with no other worthy challengers with better resumes, the Tide and the Tigers take the Iron Bowl to Pasadena.
Crazier things have happened.