You develop claustrophobia, fear of water, etc... either legitimately, or convincingly malingering. I saw it twice on the sub... and you know what- good riddance to them. For each of them, we tried to help them "back from the ledge" so to speak, until it was clear that wasn't working. Then we did the only reasonable thing- we pushed them over. Psychologically, not physically of course. At some point, you don't want an unstable person on your crew, with potentially your life in their hands.