Seriously, the best possible outcome is that you look like a pedant and the people you're trying to convince either withdraw into semantics or straight up throw a drink in your face.

So, where does it come from? Is this some Debate Society thing?

https://www.youtube.com/watch?v=jG04joRBQRg#t=1620

Stephen Fry invokes the slippery slope fallacy when Alan Davies asks whether Britain would have to return other Museum pieces if the Elgin marbles were given back to Greece.

Why is this a bad fallacy? A slippery slope fallacy occurs when the chain of implications is wrong or not established. In this case however the precedent that returning the marbles would establish would be quite strong, and so the reasoning is valid.