Definition
A slippery slope argument claims that taking one step will inevitably set off a chain of events ending in an extreme, usually undesirable, outcome. It becomes a fallacy when the chain is asserted rather than argued: each link needs its own reason to hold, and the argument skips straight from the first step to the worst-case ending.
Example
"If we let students retake one failed exam, eventually no test result will mean anything." This skips over every safeguard that would stop the chain: a limit on retakes, teacher discretion, or grading policy. The claim treats one policy change as automatically producing the most extreme possible version of itself.
Common mistakes
Not every slope is a fallacy. Some causal chains really are tight, and a well-supported slippery slope argument explains why each step follows from the last, with its own evidence at each link. The mistake is treating "one thing could lead to another" as sufficient on its own. To catch this in a round, ask your opponent to walk through the mechanism step by step; if they cannot explain why step two follows from step one, the slope was assumed, not argued.