I'm amazed no one has mentioned Cold Lake yet. There hasn't been a more catastrophic misstep in music history IMO.
Another one I thought of:
So in the early 80s, and I admit, a lot of it was probably due to relentless self promotion by the band, Manowar had a reputation for being a serious TROO metal band. They had a reputation for loud performances and being generally All Things Heavy Metal. Then they sign with a major label and released the video for Blow Your Speakers.
And there they were, Manowar, the band with a reputation for being All Things Metal, playing a hair band song that would've made every band in Los Angeles cringe.
I know Manowar is still pretty huge in Europe, especially Germany, but that song killed them DEAD in the United States.
And as for Helloween-they died when Kai Hansen left and they put out Pink Bubbles Go Ape and Chameleon, IMO. Kiske leaving was just the icing on that particular cake.