It was Brian Kelly's foolish and stubborn decision to try to turn Dayne Crist into a spread-option QB, instead of letting him be a classic dropback passer. He was putting up some good passing numbers before that awful knee injury.
It was also Brian Kelly's foolish decision to not let him at least try to redeem himself in another game in 2011. Yes, South Florida was a debacle, and he did a bad job on the 2nd drive (the first drive wasn't his fault), but to not let him have a shot at trying again was a terrible mistake. That should have been a 10+ win, BCS bowl team.
Kelly's attitude was that "there are my guys, and there are guys who were brought in by the last guy." (he considered Rees one of his own, even though Weis brought him in)
There were several of those locker room speeches of his where he would say the above, and came ever so close to chasing away Manti Teo.
Crist made a terrible decision to play for Kansas. For that matter, it was foolish of Weis to take the job there after having had a very successful season with the KC Chiefs as the OC.
There was virtually no talent on that team, especially on the offensive line.