One of the most difficult constitutional principles to teach, for reasons I really don’t understand, is the rule that in order to be constituional, a law must be sufficiently precise to allow citizens to know what behaviors will be sanctioned. If a law does not meet that standard, we say it is “void for vagueness.” (This is the problem with so many “anti-pornography” efforts; one person’s porn is another person’s erotica, as Nadine Strossen once put it.)
A good example of the problem with overbroad and vague prohibitions is the patriot act provision being challenged in this case, which the Supreme Court will hear this term.