The Tampa Bay Buccaneers had a pretty good run defense last year: Football Outsiders ranked them eighth in the NFL, and they allowed an NFL seventh-ranked 3.9 yards per carry. But it seems that as soon as opposing teams introduced a fullback into the equation, the Bucs suddenly couldn't stop anyone. According to Football Outsiders, they had the 24th-ranked run defense against two backs while their one-back run defense was ranked second in the NFL.
That's a pretty curious gap, but it could easily be explained by the fact that facing a fullback usually means lining up with three linebackers, while facing a single back usually means going to a nickel defense.