The TV show Earth: Final Conflict suffered from almost comical levels of cast churn. The main characters changed on an almost seasonal basis. Wikipedia has a handy chart showing which characters were around for each season. Wikipedia also mentions:
The show is famous for the unusually high turnover rate among the regular cast, partially due to contractual disagreements between the cast and the producers. Almost all of the show's major characters were killed or otherwise removed within a season or two of being introduced.
But the show ran for 5 seasons, and showed this problem throughout the entire run. Is there any more detailed information on why there was a constantly revolving cast on this show? Wikipedia credits the issue only partially on contractual disagreements, and doesn't have more information on the cause of this. In fact that section of the article is unsourced, so even the 'contractual disagreements' aren't confirmed.