Miranda warning
The law says the police have to give a suspect a Miranda warning before they start questioning them.  If they do not, and the suspect gives a confession, that confession could later be thrown out in court.  This means a court could rule that the confession cannot be used as evidence in a trial.  A confession can also be thrown out if the police gave a Miranda warning, but gave it the wrong way, or did not make sure the suspect understood it.  (This is discussed more in the "Problems" section.)