Sheena Easton in Miami Vice. Some of the fans argue she was the worst thing to ever happen to that show.
They introduced her in a panic as a reaction to the show jumping the shark with the infamous UFO episode (the one with James Brown). The backlash to that one episode was so bad, they figured the only solution was to introduce a new character and have her as a love interest to Crockett.
Among MV fans, Easton's character Caitlin has been debated to death. I don't care that she's a pop singer and it's unrealistic that a vice squad detective would be dating her. The show already required suspension of disbelief with how Crockett is never figured out as a cop by the Miami underworld after so many successful busts of major drug dealers using his undercover Burnett persona. I also have to bring up that this happened in season 4, which is the worst season of the show. It starts off poorly and there's plenty of bad episodes that don't involve Easton, so I don't think it's fair to blame the show's decline on her at all.
The main problem is the complete lack of chemistry Easton had with Don Johnson. She's just a poor actress in general. There's this one episode with her where Crockett is undercover searching for a female serial killer played by legendary supermodel Iman (David Bowie's wife), and she has way better chemistry with Johnson and put in a memorable performance as a villain.
There was one good thing though. Easton's character gets killed off and that being the catalyst for one of the best story arcs of the entire series with the Burnett Saga, and starts season 5 off on a big high.