Whenever reactionaries make a slippery slope argument in the public square, they’re almost always correct, although their timing is sometimes off a bit. For instance, they predicted that legalizing contraception for unmarried people would INCREASE the rates of unwed mothers—certainly very counterintuitive at the time. And…they were right. Pretty much everything they have predicted has come to pass. Slippery slope might be a fallacy in the Platonic world of formal logic, but it is an inductive argument that is usually correct when discussing human behavior, politics, or law.
I'm hard pressed to think of any counterexample.