Of course you can yell fire in a crowded theater. Perhaps you smelled smoke from the popcorn machine (or were having a stroke) and everyone would get up pretty calmly and leave the theater (or tell you to shut up and watch the movie/play). If someone twisted their ankle or even managed to get trampled to death few lawyers would even consider a case against you.

Now, if you had clear intent to cause injury, blocked exits to ensure harm, and started the fire you'd be up on several charges related to fire safety codes & arson laws long before you had a prior restraint on speech claim (i.e. 1st amendment).

The original Supreme Court case (Schenck v. US [1]) from which the rhetorical flourish about fires and theaters came [2] has been pretty much abandoned in favor of later rulings like Brandenburg v. Ohio [3].

[1]: https://www.law.cornell.edu/supremecourt/text/249/47

[2]: https://www.popehat.com/2012/09/19/three-generations-of-a-ha...

[3]: https://www.law.cornell.edu/supremecourt/text/395/444

One more interesting read on this topic: https://www.cnn.com/2017/04/27/politics/first-amendment-expl...