Shutter Island
Shutter Island had viewers flip-flopping on whether or not Leonardo DiCaprio's character was crazy or not. He gets trapped at the mental hospital he was supposed to be investigating and tries to escape. Eventually, we find out that he's actually a mental patient who is having an episode the entire time. He was suffering from the guilt of killing his wife after she had killed their children.
Barton Fink
A playwright is hired by a big studio in Hollywood to write some scripts. He becomes disillusioned with the lifestyle and eventually has a psychological break. At the end of the film, we see a woman standing on the beach. The woman looks exactly like a photo that was hanging up in Fink's room. The director explained that he wasn't actually seeing the woman, but he was hallucinating her because she represented his lifestyle in Hollywood.