Equinox said:
Now I'm gonna wonder about all the plot twists and turns and wonder if writers actually wanted it or if the higher ups just said, "Hey, we don't like person A anymore - have something bad happen to them on screen"...
There are plenty!
Grace Lee Whitney was not able to give her role 100% and so Janice Rand disappeared.
The actor playing Mr Leslie in TOS had a back injury and had to stop taking work on TOS.
Chekov got to do more in TOS because Sulu was off filming a movie that got delayed a lot.
Palmer got to play at communications whenever Uhura had a singing engagement in real life.
Chief DiFalco navigated the Enterprise home in TMP because Ilia was kidnapped by V'ger and Marcy Lafferty was married to William Shatner.
Saavik changed her eyebrows, eye colour and her whole face because Paramount offered Kirstie Alley less money for ST III than ST II.
Dr Chapel was not in ST II because her husband had lost creative control of the movies and he was threatening to withhold the Roddenberry name from the credits.
Janice Rand was in ST III because director Nimoy liked her.
Janice Rand was in ST VI because fans voted for her at a ST convention. Kevin Riley lost by a few points. And then Christian Slater got a few of her lines because... his Mom was the casting director!
Tasha Yar died because Denise Crosby didn't feel she was getting enough lines.
Dr Crusher went to Starfleet Medical for a year because Gates McFadden was claimed to be "difficult" by one of the producers.
Dr Crusher came back because Diana Muldaur refused to be available, for three consecutive Season Three episodes, so she could film a "Return of McCloud" reunion telemovie.
Guinan disappeared in TNG's Season Six because her movie career kept Whoopi Goldberg too busy to tend bar.
Scotty and Chekov accompanied Kirk in "Generations" because Spock claimed he didn't have a significant part, and McCoy felt he'd already said farewell properly in "Encounter at Farpoint".
Ro Laren lost a job on DS9 because she wouldn't sign a longterm contract.
Jadzia Dax was killed by Gul Ducat because Terry Farrell had been leading a campaign for higher cast salaries on DS9. The producers ended up approaching each other cast member separately, offering better conditions, thus breaking the alliance and leaving Dax out to dry.
Uhura didn't get to talk to Sulu and Rand in "Flashback" because she wanted a walk-on role, not just a viewscreen scene.
Mrs Troi terrorized the 24th century because Majel Barrett was married to the Great Bird of the Galaxy.
And so on.