Bruce Arians is a great example. You could not find a Bama fan after the Awbarn mishap/loss that would have hired Arians to coach their local HS team. Turns out guy was a pretty darn good coach and one heck of an NFL head coach.
I'm not entirely sure why, but people tend to have extreme recency bias, as well as being easily influenced by the halo or horns effect. People are easily mislead by a failure to take everything into account.
I'm not inclined to do a deep dive on Arians at the moment, but the idea that a single mishap turns him into a bad coach is complete and utter nonsense. But some people can't see past that.
Another example of recency bias clouding people's judgement would be Norvell. Earlier this year I even had a discussion where someone was like, yeah but everyone thought he was a good coach. Why? One good season? That's nuts. Yet, somehow that made people forget he'd just gone 18-16 at FSU (not exactly a tough place to win) and allegedly even pushed him to the top of the candidate list for the Alabama job.
Leigh Tiffin is another good example, he's one of the best kickers Alabama has ever had. But he had that one bad game against Arkansas, where he was foolishly sent out there over and over despite obvious issues and a lot of people wrote him off. All they could remember for some time was that single game and it clouded their judgement.
Even with the Kiffin hiring, a lot of people had a negative view of him based on a few things that had occurred and basically swore up and down he'd be a bad OC. He had his downsides, he still does, but he certainly wasn't a bad OC.
It's just hard for people to see the big picture quite often. I don't think that's necessarily a reflecting of reality though, you can be objective a lot of people just have difficulty doing so. They tend to seize on one thing and dismiss the rest (he did well at Washington, or Oregon, or conversely he did poorly at USC or Alabama for example, and don't look at the entire resume for some reason).