Shout fire in a crowded theater. Its literally the first example in that you aren't allowed to say anything you want whenever you want. You only have protection against government retaliation.
In the US, shouting fire in a crowded theater is expressly allowed per Brandenburg v Ohio in 1969. It puzzles me why it is so often trotted out as an example of things you can’t say since we had a whole Supreme Court case that determined the opposite — it is arguing the losing side. The kind of speech that is disallowed in the US is very narrow, much narrower than people apparently assume.
Or at least you're supposed to