What is it?
A slippery slope fallacy occurs when someone argues that a relatively small first step will inevitably lead to a chain of related events culminating in some significant (usually negative) impact, without establishing a causal connection between the first step and the outcome.
Examples
Person A: 'If we allow same-sex marriage, next people will want to marry their pets!'
Person B: 'If we ban this type of gun, eventually the government will take away all our weapons.'
How to Avoid This
When making predictions about consequences, ensure there's a logical and probable connection between each step. Avoid making extreme predictions without evidence for the causal chain.
How to Counter This
Challenge the assumed inevitability of the chain of events. Ask for evidence that each step would necessarily lead to the next, or point out intervening factors that would prevent the predicted outcome.