 The opacity of many computer systems can get in the way of assessing the validity and relevance of the information and can prevent a user from making appropriate decisions. People have a tendency to either rely too much or not enough on the accuracy automated systems (Cummings 2004; Parasuraman &amp; Riley 1997). A person’s ability to act responsibly, for example, can suffer when she distrust the automation as result of a high rate of false alarms. In the Therac 25 case, one of the machine’s operators testified that she had become used to the many cryptic error messages the machine gave and most did not involve patient safety (Leveson and Turner 1993, p.24). She tended ignore them and therefore failed to notice when the machine was set to overdose a patient. Too much reliance on automated systems can have equally disastrous consequences. In 1988 the missile cruiser U.S.S. Vincennes shot down an Iranian civilian jet airliner, killing all 290 passengers onboard, after it mistakenly identified the airliner as an attacking military aircraft (Gray 1997).  The cruiser was equipped with an Aegis defensive system that could automatically track and target incoming missiles and enemy aircrafts.  Analyses of the events leading up to incident showed that overconfidence in the abilities of the Aegis system prevented others from intervening when they could have. Two other warships nearby had correctly identified the aircraft as civilian. Yet, they did not dispute the Vincennes’ identification of the aircraft as a military aircraft. In a later explanation Lt. Richard Thomas of one of the nearby ships stated, “We called her Robocruiser… she always seemed to have a picture… She always seemed to be telling everybody to get on or off the link as though her picture was better” (as quoted in Gray 1997, p. 34). The captains of both ships thought that the sophisticated Aegis system provided the crew of Vincennes with information they did not have.