Fake seizures are a popular but predictable option. The act ranges from half-hearted shivering, to full-blown convulsions to be followed by status dramaticus. While peeing your pants to simulate incontinence may merit an A for effort, it's rarely convincing and just leaves you in wet undies.
Also common is "incarceritis," the mysterious constellation of symptoms including sudden onset severe chest pain, sudden onset severe abdominal pain, or sudden onset severe crying "I need to see a doctor."
While these acts may buy patients a couple hours in the ER, they almost always end up with a quick return to jail. Last night, however, one prisoner/patient was so determined to avoid spending the night in the slammer, he banged his head violently against the door of the ambulance as he was escorted to the hospital and ended up with a small skull fracture. He ended up buying himself a night in the hospital - in police custody of course - only to end up in jail as soon as he was discharged.