Shanahan's final game with the Falcons, Super Bowl LI on Sunday night before he stepped away to become the presumptive head coach of the San Francisco 49ers, was everything the Falcons and their fans -- and the Niners -- could have wanted.
And then, wham, it became nothing but a punch in the gut.
It wasn't so much the 37-year-old Shanahan, about to become an NFL head coach for the first time in his career, was walking out the door. Rather, the door was slamming on his backside.
Atlanta never scored again, and in the first overtime in Super Bowl history, the Patriots suddenly were 34-28 winners.
The charming little tale involving Shanahan and a journalist who accidentally picked up Shanahan's backpack Monday at media night was no longer so charming.
The backpack was never opened by the sports writer, so the Falcons' game plan in there went unseen. But Shanahan, who could have wiped out the plan on his laptop with a push of a button -- even if someone else had possession of the computer -- was unhappy not because of unintentionally pilfered X's and O's. There were tickets to the game in the backpack.
For a while Sunday night, it seemed as if the game plan had some magic, too, as Atlanta went in front with Devonta Freeman running, Matt Ryan throwing and Julio Jones catching. Then, poof. The magic was gone.
Not because Shanahan became conservative after halftime, according to Ryan -- the Falcons even had the ball to start the third quarter -- but because there were missed opportunities.
"I thought Kyle did a good job for us tonight," said Ryan, announced Saturday as the league's Most Valuable Player. "We made a few mistakes, and I think when we got their onside kick (in the fourth quarter), we had a short-field opportunity and didn't come away with any points."
The old "if it ain't broke, don't fix it" idea is something Shanahan and Quinn both endorse.
"I believe in sticking with what we have been doing," Shanahan said Thursday. "You have to. Everyone kind of just does what they do. You don't want to reinvent the wheel."
Instead, the Falcons invented a way to blow a Super Bowl. And it wasn't necessarily because of play-calling. The way virtually everything worked the first half, nothing worked the second.
"We got sacked, and that put us out of field-goal range," Shanahan said about a critical sequence. "Then we got a holding call and were out of it again."
Now, about to join new general manager John Lynch with the 49ers, Shanahan is out from the Falcons. But with some regrets about the Super Bowl.
"I always want to run the ball more," Shanahan said, "if you can. You have to look at each situation, when you're getting the ball, the down and distance. (In the second half), we got a few big plays in the pass game, missed a couple in the run game.
"Had a couple guys go down, a running back, a tackle. We got behind the chains a little, and we were trying to score. The Patriots went to a little bit more man-to-man coverage."
Even with Atlanta ahead from the opening minutes of the second quarter until the last minute of regulation, the Patriots had a huge edge in time of possession. New England had the ball 19 minutes the first half, 17 the second and the entire 3:58 of overtime.
Atlanta was 1 of 8 on third downs in the game.
"I'll go back and watch it a lot," Shanahan said. "I'm sure I'll have a better answer. There is no one reason we didn't get it done."
But the way the Falcons played in the first half is the reason the 49ers chose Shanahan, with or without his backpack.