Not quite correct. You are not allowed to yell fire in a movie theater if there is no fire and the yell is likely to cause imminent harm. It's a "myth" only in

Not quite correct. You are not allowed to yell fire in a movie theater if there is no fire and the yell is likely to cause imminent harm. It's a "myth" only in the sense that this simple rule (can't use speech to cause physical harm) is use in an attempt to justify things that are not imminent. For example, in the infamous Supreme Court ruling, it was used to justify banning Jehovah's Witnesses from distributing pamphlets encouraging people to lobby the government against the draft. That was an incorrect and stupid use of "shouting fire" because the speech does not cause imminent harm, nor does it cause physical harm except through legally changing public opinion. So it's a "myth" only in the sense that it is a sure marker of a dishonest argument: if someone uses it in speech, they are almost certainly using it to try to justify something which is not the same thing at all.