Yelling fire in a movie theater is perfectly legal and protected speech, even if false. It's only illegal if what is said is "likely to incite imminent lawless action," like a riot.
https://en.wikipedia.org/wiki/Shouting_fire_in_a_crowded_the...
It's funny, this is often presented as a supporting example to limit citizens' free speech or other rights, typically paired with something along the lines of, "no freedom is limitless." Of course many people don't realize this example is a) outdated and currently false, and b) the argument used against citizens speaking out against WWI using the Espionage Act of 1917, which is considered by many to be one of the worst and oppressive laws to our rights ever written.
https://en.wikipedia.org/wiki/Espionage_Act_of_1917
You may recognize the names of some of the act's victims:
Among those charged with offenses under the Act are German-American socialist congressman and newspaper editor Victor L. Berger, labor leader and five-time Socialist Party of America candidate, Eugene V. Debs, anarchists Emma Goldman and Alexander Berkman, former Watch Tower Bible & Tract Society president Joseph Franklin Rutherford, communists Julius and Ethel Rosenberg, Pentagon Papers whistleblower Daniel Ellsberg, Cablegate whistleblower Chelsea Manning, WikiLeaks founder Julian Assange, Defense Intelligence Agency employee Henry Kyle Frese, and National Security Agency (NSA) contractor and whistleblower Edward Snowden.