When do Individual morals supersede professional expectations [elaboration within...]?

I know that psychiatrists and psychological professionals, as well as lawyers, have a certain responsibilty to keep the information their clients share confidential. If there were a client speaking of something harmful, such as a planned attack on citizens, etc., is it acceptable/legal for the professional to notify authorities?

Would the professional be at fault here, legally? Of course it’s the “right” thing to do, but is it? Can you think of an example when this would be acceptable and when it wouldn’t be?