[Husker] Roughing penalty

[Monty Perry]

Anyone know why, after the roughing penalty at the end of the first half 
when Zac got clocked, that the officials spotted the ball back on the 20 
yard line and the TV showed a first and 20? I replayed it and sure enough, 
there was a flag laying on the ground, but I heard no explanation why the 
ball was back on the original line of scrimmage.

Maybe something was said on the radio broadcast?
Thanks,

Monty