It may be some quirky ideas, the meddling of a Corrupt Corporate Executive or other villain, having noticed something Invisible to Normals, an attempt to dodge jail by getting a jury to call Insanity Defense, or just a good old fashioned screw-up, but somehow a reasonably sane character undergoes severe therapy, often in an asylum. Despite their relative normality, they struggle to convince the therapists of the fact. And the more they struggle, the closer they get to lunacy itself, sometimes becoming indistinguishable from the others around them. See also Had to Come to Prison to Be a Crook.