Well, no, it's dicta in the textbook case—Schenck v. United States, 249 U.S. 47 (1919)—of the Supreme Court getting swept aside by security hysteria and failing to protect the First Amendment; a decision whose substance was overruled by Brandenburg v. U.S., 395 U.S. 444 (1969).

I stand corrected, thank you. I recall being told specifically in 7th grade social studies that that was an actual case in the 1920's where someone did yell fire. I'm aware of other cases where the court decision was, "here's a ridiculous extreme we obviously wouldn't go to, so obviously let's just rule that it doesn't apply at all," and I hate it, so I'll stop using this :)