There are also a host of other well-respected exceptions to free speech, like shouting "fire" in a crowded theater.

I wonder how many people would use this example nowadays if they knew that it comes from a WW1-era U.S. Supreme Court opinion upholding a ten year prison sentence for sedition against an anti-war activist -- whose crime was to distribute pamphlets arguing that military conscription is unconstitutional under the 13th Amendment, which prohibits "involuntary servitude."

(By the way, speaking of slippery slopes, the following year the same court upheld another ten year conviction for a speech whose content was carefully crafted to remain within the bounds of the sedition laws, but which was still judged to be illegal on the grounds of intent and indirect implication.)