A dentist client told me last week that if a new patient shows up with a zip-lock baggie containing half-a-dozen sets of old dentures, it signals the beginning of a rough relationship. The patient is wordlessly communicating to the dentist that no other dentist has ever made him happy and it is now his turn to prove that no dentist ever will.
These are the customers/patients who will sap your energy with whining, drain your valuable time and resources and then complain about you after they have finally defeated you and found their next victim/vendor/doctor.