The phrase “fire in a packed theater” in the legal context is actually based on a case where someone was jailed for peacefully protesting the draft for WW1 (Schenck v. United States). It has no relation to actual violence, and is in fact a fantastic example of how “harm” by speech can be abused by a malicious government to jail people for what should be protected speech.

It’s no longer good law, thank goodness. Now we have the much narrower Brandenburg test, which requires that speech be likely to cause “imminent lawless action”.