The Low-Fat Diet
How it REALLY works: Conditioned anorexia.
You are required to eat at least 6oz of protein every day, but the only allowable forms are lean skinless chicken breast, or lean skinless turkey breast, with no gravy, sauce, or butter. After a week, you are disgusted by the taste of poultry breast. After two weeks, it all tastes like cardboard to you, and you dread eating it. This dread generalizes, and you become apathetic about eating at all. So you mostly don't eat. You become depressed, the passing of days becomes hazy, and you only eat when you blearily remember that you aren't quite ready to die yet.
How it fails: Free Will.
One day, a thought will bubble up through the fog of your starving brain. You will realize that you have only been considering two options: eat more dry, bland, flavorless chicken, or slow starvation. But there is a third choice! There is a way out! You can STOP SUFFERING and EAT NORMAL FOOD! Crying with the joyous revelation, you eat something deep-fried and covered in gravy. You nap the contented nap of someone who is actually satiated (the best sleep you can remember), and wake up able to think clearly for the first time in weeks. You decide that any weight you lost on the diet was good enough for now, and you'd rather be not-quite-at-goal-weight and HAPPY than thin, miserable, depressed, and incoherent.