There's a section of that page dedicated to non-fallacious usage. If a decision makes a subsequent decision more likely, the former is a slippery slope to the latter.

There is very little risk that I hurt myself doing cocaine responsibly once. The risk in doing cocaine is that if I do it once, I will be inclined to do it again and so on until I'm a broke addict. If slippery slopes were a mythical problem, I could just commit to doing cocaine only once.

If you ban alcohol, private citizens will be more likely to decide to start a criminal enterprise to supply it outside of the law. If someone is running a profitable criminal enterprise, they'll be less concerned about breaking the law in general and more likely to branch out to violent crimes. If slippery slopes were a myth, we could institute prohibition without risking a massive expansion of the criminal underworld.

I can't find it now, but I saw a comment on HN the other day where a UK citizen expressed how foolish Americans were for disliking their knife laws. Of course they should ban knives in public- knives account for the majority of homicides in their country. No one in the US could be convinced by that argument until effective gun control made it true. Instituting gun control makes it more likely for citizens and politicians to support knife control. (Whether this is worth it is another matter.)

Independent decisions are a myth; every decision is made in the context of the ones that came before.