from:
Is it illegal to yell "fire" in a crowded theatre?
"The origin of the phrase is from the Supreme Court of the United States in the case Schenck v. United States, 249 U.S. 47 (1919). It specifically rules on the
limitation of freedom of speech (first amendment):
The original ruling is this:
The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre 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.
As pointed out by @phoog, this does not saying anything about the lawfullness of shouting "fire", it says that if your speech
creates a clear and present danger, the first amendment will not protect you, even if the danger does not result in actual harm."
So my example is accurate, with a caveat.
"...it says that if your speech creates a clear and present danger, the first amendment will not protect you, even if the danger does not result in actual harm."