"It" being a total reversal of performance quality from the first half to the second half against Jacksonville State.
"We went from real good to real bad quickly," D.T. said, shaking his head. "It was tough to watch and be a part of it. We had a spirit of being satisfied at halftime. We fell in the trap of thinking we had it won.
"Our coaches were preaching to us about going on and getting them out of the way, keeping the intensity high and burying them, but we didn't listen. We let them off the mat, they got the momentum and then we couldn't stop them. We made a terrible mistake."
Saturday night and Sunday, Shackelford said, were "horrible."
"I talked to my Mom and Dad, who came in for the game, then I muted my phone Saturday night. I didn't want to talk to anybody," he explained. "It was a horrible feeling.
"Sunday, we came in and tried to get that one out of our system, but it was hard. I can tell you, we learned a hard lesson. We won't have another breakdown like that again. It's all on us. I credit Jacksonville State, they never gave up, but I'm disappointed that we weren't able to respond even after we saw the game slipping away.
"We underestimated our opponent and it cost us. We won't make that mistake again. It's too late to correct that one, but we won't let it happen again. We will never get complacent again. We played hard for three quarters and then just gave in during the fourth quarter thinking we had it won. It was bad. Our expectation level is so high that this is by far the most disappointing loss I have ever been through."
Shackelford played some MLB and some defensive end last weekend.
"Whatever I can do to help the team, I'm going to do. With (Kentrell) Lockett out, the coaches asked me to come off the edge some. I did it in high school a lot and enjoyed it," he explained.
MLB Jonathan Cornell didn't see it coming, but as it was unraveling he kept encouraging his teammates to tune it back in.
"All you can do it to keep playing hard and try to make something happen, but we got out of synch," Cornell stated. "We were not on the same page with our rush or our coverages.
"We had a lot of mistakes and that was kind of shocking considering we have a lot of veteran players on the defense. Defense is all about 11 guys working together and doing their jobs. We had too many mental breakdowns at key times. Everything is interdependent on everything else and we just didn't get it done. There was no one area that deserves blame - we are all to blame."
Being the veteran, and having been through just about every up and down possible in his Rebel career, Cornell said he took the business approach on Sunday.
"You can't get that one back. As bad as you would like to have a do-over, you can't get one," he explained. "The only thing you can do is put that one behind you, learn from it and go back to work.
"We have a lot to prove to ourselves and to everyone who cares about us. I'm not looking for redemption, I'm looking for us to play our game. We didn't play our game Saturday and it cost us. If we play our game and lose, I can live with that, but I can't swallow not playing our game."
It's been a bitter pill for D.T. and Jonathan, but it's one they are determined not to take again.
"We'll get this bad taste out of our mouths and move on," Cornell closed.