Body dysmorphic disorder
Body dysmorphic disorder (also known as "dismorphophobia") is an extremely rare disorder wherein the afflicted are perpetually unhappy with their own body image, and often perceive themselves in an idiosyncratic fashion that may be inconsistent with reality (for example, an 85 lb. woman who still perceives herself as "chunky"). Generally, people think that there is something wrong with their appearance, often based on some perceived minor defect. These people will seek repeated plastic surgery and sometimes non-mainstream body modifications in an effort to change the appearance of their bodies to match their expectations. However, because this problem is delusional in nature, the "destination body" is never found.
BDD, as it is often called, can actually cause the afflicted person to physically see themselves as being disfigured in some way (for instance, a thin person may not just think "I am fat," but will actually look in the mirror and physically see fat that is not there). BDD is often associated with eating disorders such as anorexia or bulimia.