Pet Peeve
A "slippery slope" is the name of a standard fallacy. It is a failure in reasoning, an error one commits in argument. You are guilty of it when you maintain that one occurrence/state would or will lead to another (usually more serious and clearly undesirable) event without arguing for why it would do so, and often skipping over many obvious necessary steps in between that would not necessarily follow. The "slippery slope" is generally a lazy scare tactic that is, by definition as a fallacy, unreasonable. It is a "where will it stop?" appeal that is recognized by logicians as an error.

You may not, for example, claim that promoting stem cell research will lead to the "divorce" of science and ethics. That is a bad argument. You will fail every logical reasoning class with it. And the words your instructor will write over your attempt are "slippery slope." They are the name of your failure.

So if you're trying to sneak one by, the last thing you want to do is to use that very name as the vehicle for your bad argument. Why do so many people use "slippery slope" as if that phrase describes something logical and truly scary that helps their point, when it is the very title for their faulty reasoning? White House spokesman Scott McClellan arguing against the danger of stem cell research: "It's important that we not go down a dangerous, slippery slope where we divorce ethics from science."

There is no demonstrated slope between the two. Certainly no slippery one. It makes me crazy. Let's not use the phrase "slippery slope" unless we're accusing someone else of being guilty of it in their own rhetoric. Tell them their failure has a name, and that is it. It is "dangerous" only in arguments, not in real life. You can read more about it, and other fallacies here. Don't even get me started on using the phrase "begging the question."