The Blackhawk debacle is an obvious frontrunner in this category.
I also nominate the destruction of Doom Patrol with the introduction of Mento and Beast Boy and the hipster Robotman.
The change-over of Challengers - a limitless concept!! - into Welcome Back Kotter with Rocky Davis' transition into a beatnick fifteen years after it was passe.
The transition of Hawkman from a nifty science fiction strip into a superhero-versus-monsters strip.
The hunted Metal Men.
The human Metal Men.
Andru and Esposito on Flash was certainly a mistake.
Kanigher's "Golden Age" Wonder Woman period - Andru trying to duplicate Harry Peter!!!
Sekowsky trying to ape Kirby in Justice League. He was a master and didn't need that grief.