Arguing without adequate evidence that one event will lead to a chain of events, culminating in a disastrous final result. It's a fear tactic that relies on assumptions rather than evidence.
It's possible to have a "logical slippery slope" if you are able to provide sufficient reasons to believe each step in the chain of events is likely to follow.
EXAMPLES OF ILLOGICAL SLIPPERY SLOPES
1. "If you become an atheist you'll abandon morality, start selling drugs, murder the people who owe you money, then spend your life in prison. If you don't like prison, don't become an atheist."
2. "If that president is elected taxes will go up. If taxes go up we'll have to work longer hours to pay our taxes. If we work long hours our health will deteriorate and we'll die at a young age. Would you really vote for a president that wants to shorten your life expectancy?"