In several cases, trespass laws have been used to charge people who illicitly enter another person's computer system without permission. The typical scenario involves putting a trace on a telephone line that enters the computer system and a method of detecting the attack on the computer. The police observe the entire action and then make an arrest. This often creates problems if the computer system is not adequately protected. The prompt for the user to use the system may be considered an invitation for entry. The computer system must often be restricted from use for a period of time in order to use information in it as evidence, and this often costs more than is justified for the action. This sort of law has been relatively unsuccessful in the prevention of illicit access, and has only resulted in jail sentences in cases involving 'national security information'.