I always love these "Mack (or any coach) won't let that happen again". Did anyone actually watch the game? Sam Howell hit Josh Kaindoh, a Defensive End, in the chest with a pass and he returned it for a touchdown. That was Mack's fault?
UNC played a terrible first half and FSU got some incredibly lucky plays. You don't see defensive ends return interceptions for touchdowns very often. FSU also blocked two punts in the half. Don't see that happen often either. It was a fluky game, it happens. It's the reason why it's incredibly hard to go undefeated in college football, and maybe 1 team does it per year. You just don't play your best week in and week out.
FSU had about 120 yards of offense in the 2nd half and were outscored 21-0. Just a weird game. They happen.