"Shouting fire in a crowded theater" is a frequent misquoting of Oliver Wendell Holmes, Jr.'s opinion in the United States Supreme Court case Schenck v. United States in 1919. The misquote fails to mention falsely shouting fire to highlight that speech which is merely dangerous and false which can be distinguished from truthful but also dangerous. The quote is used as an example of speech which serves no conceivable useful purpose and is extremely and imminently dangerous so that resort to the courts or administrative procedures is not practical and expresses the permissible limitations on free speech consistent with the terms of the First Amendment of the United States Constitution.
The Schenck case
Holmes, writing for a unanimous majority, ruled that it was illegal to distribute flyers opposing the draft during World War I. Holmes argued this abridgment of free speech was permissible because it presented a "clear and present danger" to the government's recruitment efforts for the war. Holmes wrote:
"The most stringent protection of free speech would not protect a man falsely shouting fire in a theater and causing a panic. <...> The question in every case is whether the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils that Congress has a right to prevent."
Holmes wrote of falsely shouting fire, because, of course, if there were a fire in a crowded theater, one may rightly indeed shout "Fire!"; one may, depending on the law in operation, even be obliged to. Falsely shouting "Fire!" in a crowded theater, i.e. shouting "Fire!" when one believes there to be no fire in order to cause panic, was interpreted not to be protected by the First Amendment.
The First Amendment holding in Schenck was later overturned by Brandenburg v. Ohio, which limited the scope of banned speech to that which would be directed to and likely to incite imminent lawless action (e.g. a riot). The test in Brandenburg is the current High Court jurisprudence on the ability of government to proscribe speech after that fact. Despite Schenck being limited, the phrase "shouting fire in a crowded theater" has since come to be known as synonymous with an action that the speaker believes goes beyond the rights guaranteed by free speech, reckless or malicious speech, or an action whose outcomes are blatantly obvious.
http://www.absoluteastronomy.com/topics/Shouting_fire_in_a_crowded_theater