2. Desmond Ridder
Let's revisit the fact that not only did the Falcons opt to draft Desmond Ridder they also signed Marcus Mariota and believed the veteran was the better option. The Falcons had Ridder in camp and practice and believed Mariota gave the team a better chance to win. With the results Mariota was having this is about as damaging as possible. Out of pure desperation, the team gave Ridder a chance at the end of the season and the quarterback wasn't awful.
It was hard to judge exactly what Ridder could be but there wasn't anything eye-popping about the first games of his career. Still, Atlanta had so much faith in a player they kept benched they put the entire fate of the next season in his hands. There wasn't a bridge quarterback brought in or another draft pick spent to find an answer. The quarterback was given the team with absolutely no capable backup plan in place.
When it went horribly wrong their answer was to put Taylor Heinicke in and out of the lineup with both quarterbacks consistently imploding. You had over a year to evaluate Ridder and put the fate of a season in his hands. This is as damaging a decision as a front office can possibly make.