People often argue some speech is unprotected by analogizing to shouting fire in a crowded theater. (Here’s the most recent example in the comments.)
But of course shouting fire in a crowded theater is often constitutionally protected. For instance, if there is a fire, shouting fire may be good. (It may sometimes not be good, if more people die in the panic than would have died from the fire if one had spoken more calmly — but even then, I’m pretty sure that it would be constitutionally protected.)
And in fact the line from Justice Holmes in Schenck v. U.S. is “The most stringent protection of free speech would not protect a man in falsely shouting fire in a theatre and causing a panic.” That “falsely” is what’s doing the work, both in Justice Holmes’s hypothetical, and in how such a false shout would be treated by First Amendment law today: Knowingly false statements of fact are usually constitutionally unprotected, whether because they constitute libel, fraud, perjury, and false light invasion of privacy; that would presumably apply to knowing falsehoods that cause a panic.
If the statement is not knowingly false, though, the analogy breaks down. The comment I linked to, for instance, was “Given the history and probabl[e] reactions, how is yelling ‘Allahu Akbar’ in a Jewish wedding different from yelling fire in a theater?” If the argument is that the speech may lead to violence against the speaker, then the arguer needs to discuss the heckler’s veto cases. If the argument is that the speech will be misperceived as a terrorist attack, so the panic may indeed flow from a false perception on the listener’s part, the arguer needs to explain why the perception is indeed likely and why the speech may be punished even though there’s little reason to think that the speaker knew the speech would be thus misperceived (or even knew there was a high probability of misperception). If the argument is that the speech may be seen as threatening, the arguer has to explain why it passes the threshold required by the threat exception to the First Amendment. But a casual analogy to “shouting fire in a crowded theater” doesn’t get us very far.