Blackburn replacing Sam Allardyce with Steve Kean. I've talked with a player he had at the time and for me there's no doubt the PL has never had a worse manager.
I think that examples such as Clough at Leeds or Hinshelwood at the Albion shouldn't really count as there had to be a managerial change. What is really bad is when a change isn't really necessary, eg Horseface at Leicester. And what's really, really bad is when a manager is doing well and then replaced, eg Zola and Rooney at Birmingham. I think to replace a manager who's performing well with an absolute duffer is an appalling misjudgment, but to do it twice is mind boggling,
When cooper left Leicester they were 16th with 10pts from 12 games but hadn't won in 5 games where they conceded 14 goals
Since then ruud horsey face has 7 points from 18 games with 4 of them in his 1st two ( so 3 points from 16).
It's awful but I do think the rooney and zola to brum ones were worse as they were a better team before the switch whereas Leicester were already in free fall albeit I still reckoned cooper would have kept them up.