Given Alabama's dominance the past four years, taking the Crimson Tide against the field to win the national title seems like a safe bet. But that doesn't mean Alabama is invincible. SI.com's Stewart Mandel and Andy Staples envisioned four scenarios in which the Crimson Tide might get derailed on their quest for a third consecutive national title. Click the corresponding image below to read about each matchup. For more college football preview content, check out SI's 2013 schedule guide, preseason All-America Team, media roundtable, lists of impact freshmen, potential BCS sleepers, teams with holes to fill and much more.