The Glass Delusion was a psychiatric disorder of the late Middle Ages in which people believed they were made of glass.

Sufferer could believe or claim that he was any sort of glass object. A 1561 account reported a sufferer "who had to relieve himself standing up, fearing that if he sat down his buttocks would shatter... The man concerned was a glass-maker from the Parisian suburb of Saint Germain, who constantly applied a small cushion to his buttocks, even when standing. He was cured of this obsession by a severe thrashing from the doctor, who told him that his pain emanated from buttocks of flesh."

Thanks to Kate for this one.