We've all heard the classic maxim that you can't cry fire in a crowded theater. To be more precise, you can't do it if you know there really is no fire.
The maxim actually comes from Oliver Wendell Holmes, the U.S. Supreme Court justice who wrote the 1919 opinion in Schenck vs the United States. Holmes used the example to show that some forms of speech can be punished in spite of the First Amendment.
False alarms are statements that are both untrue and likely to cause harm. Therefore they don't fall within the umbrella of protected free speech.
California law addresses just such a situation. Found in Penal Code 148.4, causing a false fire alarm is a misdemeanor. It's not only illegal, it can send someone to jail for up to six months (longer if a person gets hurt or killed in the ensuing response).
Presumably there is no legitimate reason why someone would sound a false alarm, other than to create trouble. And false alarms can cause significant social and economic harm, not the least of which is fear and panic among people in the vicinity.
As you might imagine, it is a defense to Penal Code 148.4 if the person entertained a good faith belief that there really was a fire. A person may call 911, for example, thinking she smells smoke…when really it's just a neighbor having burnt some bread.
As a matter of public policy, it's important that honest mistakes be excused, even if the person over-reacts to the perceived signs of a fire. The last thing we would want to do is inhibit people from reporting dangerous situations.