Biggest own goals in redistricting

[Skill and Chance]

The 2010s map for Arkansas was drawn by Democrats intending to be competitive in three of the four seats, only for them to end up not winning any of these seats from 2012 onwards due to the overall rightward shift of the state. Had the Democrats drawn a district that stretched from Little Rock out east toward the Mississippi Valley, they would likely have held on to that particular seat for the entire decade, even with the statewide shifts.

It gets worse. Had they drawn a competent gerrymander, they probably could have averaged 1.8 seats over the course of the decade.

Several high profile cases of Blue Dog Dems in denial between 1990-2010.  The GA 1991 and TN 2001 maps were similarly overconfident, but the downfall wasn't quite as instantaneous.

[Skill and Chance]

Not sure if this counts but if the national environment has been a tiny bit better for Democrats a number of Republican decisions could have been the difference – they could have nuked KY-03, MO-05, IN-01, one of the NH seats, maybe even GA-07 and/or FL-23. Might have run into issues with the courts in KY and MO but still.

Kentucky has some pretty strict laws on redistricting that would make getting rid of KY-03 basically impossible, and some internal political realities that would make changing the law almost as impossible.

Yes, the KY rules/judicial traditions are on par with Fair Districts Florida.  KY-03 fits within one county, so it has to stay there, and there isn't anything R enough anywhere in Jefferson to make it interesting.