Charlie Sheen and Selma Blair
These two starred in Anger Management together and never really got along. Blair says that Sheen's work ethic was poor and hindered her own. Sheen said that Blair had constant complaints about his unprofessionalism, which he found to be unprofessional. Sheen ended up having her written off the show.
Nathan Fillion and Stana Katic
The Castle co-stars supposedly never got along, but never made it known. A source claimed they went through seasons of not saying words to each other outside of filming. When Katic announced that she was leaving the show, fans had no idea that the decision was made in bad blood.