8. Mandy Patinkin (Jason Gideon) - Criminal Minds
For Criminal Minds' first two seasons, the great Mandy Patinkin played legendary FBI profiler Jason Gideon, and despite receiving strong acclaim for his performance, he ultimately decided to leave out of nowhere at the start of season three.
Literally, Patinkin didn't show up for a planned script read and ghosted the cast and crew for an entire week, before it was revealed he would be leaving for "creative differences."
Gideon therefore had to be hastily written out, the character venturing off to find himself in a scene Patinkin later agreed to shoot, while the rationale for his swift exit only became clear in later years.
Patinkin called it quits due to the show's abundance of violent content, a decision he assumed would mean he'd never get hired for TV work again.
Though of course, he landed on his feet some years later when he won the career-defining part of Saul Berenson on Homeland.
Still, that Patinkin bailed out in such hurried fashion is extremely uncommon and was therefore enormously jarring to both those in charge of the show and fans who loved his character.