Raffi Melkonian points to the English proposal to ban incitement of religious hatred as an example of a slippery slope (what I call the "equality slippery slope"):
The British home secretary, David Blunkett, has proposed a ban on speech that incites religious hatred. The law would obviously be unconstitutional in America, I think, since it throughly fails the Brandenburg test. But far more astonishing is one of Blunkett's arguments in favor of the ban. As he puts it today in the Observer,
For example, how can a modern society say Jews are protected (rightly, because they are covered by race laws, rather than religion), yet Muslims and Christians are not? Can it be right that hatred based on deliberate and provocative untruths about a person's religion remains unchallenged?
But this is a particularly weak argument, because it doesn't explain why laws against incitement to racial hatred (but which fall below the barrier to incitement to violence) ought to exist. And for those of us who have heard Professor Volokh's Slippery Slope talk (or read the article), I can't think of a more paradigmatic example of how one undesirable law can be used to faciliate the passage of another one later.
For more on equality slippery slopes, see here (especially starting at p. 17). One way of thinking about this is "censorship envy," as some groups that might otherwise tolerate offensive speech demand its restriction when they see that speech which is hostile to other groups is restricted. (See my explanation of why this should lead us to resist calls to ban flagburning.) Or one could focus on voters in the majority, as the equality slippery slope analysis does: If one important part of a pro-free-speech majority coalition values equal treatment of speech, then carving out one exception may lead them to swing around to supporting another exception, because their preference for equal treatment (e.g., of speech that's hostile to Jews, an ethnic group, and speech that's hostile to Muslims, a religious group) overcomes their support for free speech rights.
In any case, all this suggests that supposed free speech "extremists" or "paranoids," such as those who are (sometimes) in the ACLU, aren't paranoid at all: They are quite reasonably fearful that recognizing even narrow exceptions from free speech (e.g., for inciting racial hatred) may lead to broader ones in the future (e.g., for inciting hatred towards religions, which are after all ideologies that sometimes merit condemnation or even hatred).
UPDATE: The original version of this post said "Muslims, a racial group" instead of "Muslims, a religious group," which made no sense; I accidentally used the wrong word. If you were confused by this at first, my apologies, and I hope this update clarifies it. Thanks to David Ball for pointing out the error to me.