I know this doesn't apply, but I think one of the worst head coach hires at the time was Anthony Lynn by the Chargers. He went from being the running backs coach for the Bills, and then moved into the interim head coach job for a game after Rex Ryan got fired. He never had experience as an actual coordinator and never called plays. Then he got hired as head coach of the Chargers for basically no reason.
And then as Chargers head coach he makes blunder after blunder (a predictable outcome because he did not know what he was doing), despite the team winning 12 games and a playoff game his first year (in spite of him). Things regressed back down to the mean the next two seasons, with the Chargers winning 5 and 7 games respectively.
As a head coach he was known for persistent game management errors, some of them receiving national attention for how bad they were, blowing big leads, and embarrassing communication breakdowns and confusion.
He also had plenty of press conference blunders, frequently throwing his players under the bus and refusing to acknowledge Herbert as the starter over Tyrod Taylor, despite Herbert playing at an elite level.
Anyway, I always thought that was a train wreck of a hire and enjoyed seeing it play out like I expected it would.