One of the common tropes used by the people who want to restrict speech is to give an example of restricted speech: “you can't yell 'fire' in a crowded theater,” they smugly claim. This piece of folk legalese permeates popular culture but the simply fact is, you can yell “fire” in a crowded theater; there’s no law against it, knock yourself out. The old 1919 case that inspired that canard is often posited in Year One Law School discussions to take all the cocky newbies down a peg.

The law that actually sets the limits on free speech is 1969's Brandenburg case, which says someone cannot incite a riot or credibly threaten imminent harm on another person, particularly the president of the United States; however, even that is obviously a debatable prohibition since so many people do it. So a fire-yeller could be charged both civilly and criminally if they intentionally try to cause havoc & chaos, and someone gets hurt. That's key, their actions must have caused some kind of provable damage; simply saying they caused a little panic, fright or offense is not enough, though the anti-free speechers want it to be.