Slippery slope
Etymology
The phrase draws on the plain image of losing your footing on a slick incline and sliding helplessly to the bottom. It shares its logic with the older idiom "the thin end of the wedge." The fallacy lies not in noting a risk but in treating each downhill step as automatic.
noun phrase · /SLIP-er-ee SLOHP/
Claiming one small step will inevitably lead to an extreme outcome, without showing the chain is likely.
What it names
Spotting it separates a genuine causal risk from a scare story built on a string of unproven "and thens."
Example
If we let students retake one exam, next they’ll demand to skip finals, then degrees will mean nothing — a slippery slope with no evidence for any link.
Related idea
It differs from a false dichotomy, which fakes a lack of options rather than a chain of consequences.