Bromden's…dream…hallucination…reality…whatever that thing was when he didn't take his pills one night. It's a strange, Silent Hill esque sequence where a wall opens up to reveal a large mechanical slaughterhouse where workers move patients around on hooks and slice them open, where upon they bleed rust and ashes. In the true horror fashion, the old Chronic mental patient that Bromden saw get sliced open and mechanically bled out really is dead the next morning.
The description of the patient who was an angry lunatic before, but became an empty shell after his lobotomy. His eyes are described as being like burnt-out lightbulbs...
The rape of patients by the orderlies... just the knowledge of the patients, who are largely helpless, being forcibly raped while being completely unable to defend themselves (and this is going on with all the nurses knowing) is utterly scary.