Slipper slope is always a fallacy. It works as a metaphor, and as a rhetorical device, but the fact that it’s possible to construct a gradient from heaven to hell is not proof that each step in the gradient is inevitable.
It’s a good way to illustrate a potential bad path. But I can construct a slippery slope from commenting on HN to making bioweapons. That should not be seen as evidence that HN commenters are likely to do such a thing, and should not be used as an argument to forego (or ban) commenting on HN.
That's not correct. Many slippery slope arguments are perfectly valid and sound, namely whenever there is plausible evidence for the existence of the respective slippery slope. It can for instance be based on historical precedents, or on probabilistic or convincing balance of consideration sub-arguments for the slippery slope premise.
Whether its a fallacy or not depends a lot on how warranted the slippery slope premise is (notwithstanding other, possible logical errors that might me made in setting up the argument, of course).
Wars are sometimes caused by small acts of violence (e.g. at the border) that are mutually retributed, becoming larger and larger, until the hostilities turn into an all out war. This is sometimes called "escalation of violence." If wars are sometimes caused in that way, then there must be some wars for which a slippery slope argument concerning the escalation of violence is not fallacious.
In a slippery-slope argument, a course of action is rejected because the slippery slope advocate believes it will lead to a chain reaction resulting in an undesirable end or ends. The core of the slippery slope argument is that a specific decision under debate is likely to result in unintended consequences. The strength of such an argument depends on whether the small step really is likely to lead to the effect. This is quantified in terms of what is known as the warrant (in this case, a demonstration of the process that leads to the significant effect).
No longer slandered as a fallacy in the lede :) the 2019 version:
A slippery slope argument (SSA), in logic, critical thinking, political rhetoric, and caselaw, is a logical fallacy in which a party asserts that a relatively small first step leads to a chain of related events culminating in some significant (usually negative) effect.
It’s a good way to illustrate a potential bad path. But I can construct a slippery slope from commenting on HN to making bioweapons. That should not be seen as evidence that HN commenters are likely to do such a thing, and should not be used as an argument to forego (or ban) commenting on HN.