"Fire in a crowded theater" was originally a strawman introduced by the Supreme Court to justify their ruling in Schenck v. United States. To remind, Schenck was a Socialist Party member who was distributing flyers encouraging resistance to the draft during WW1, and was convicted for the same under the Espionage Act of 1917.

SCOTUS affirmed his conviction, saying that "the words used are used in such circumstances and are of such a nature as to create a clear and present danger that they will bring about the substantive evils".

Here's the flyer itself, in case you want to read those very dangerous words for yourself: https://en.wikipedia.org/wiki/Schenck_v._United_States#/medi... https://en.wikipedia.org/wiki/Schenck_v._United_States#/medi...

So, all in all, a good reminder that not only the slippery slope very real, but sometimes it's there from the get go.