The Tarasoff rule is a law that requires psychologists and other mental health professionals to warn people of threats made against them (and take other protective actions ) by clients. Although this appears to be a violation of client confidentiality, it is mandated as a means of making people aware of threats against them. This Supreme Court made this ruling in 1974, with a revision in 1976, due to a case where a mental patient had made threats against an individual which were later carried without the victim having been aware of the threats or danger.
Also see Duty to Warn.