The idea of a "slippery slope" is based on real and provable patterns of behavior, which is a valid thing to draw conclusions from. Think Lucy, Charlie Brown, and the football.

pwokenyaw correctly points out such patterns in anti-abortion groups and anti-gun groups, although he seems to think they aren't important. There are similar patterns in the environmental movement, in those who would like America to be socialist, and in others who have their own axes to grind.

In each case, a benign proposal is inevitably followed by a slightly less-benign one, in the classic "boiling frog" strategy. Or maybe the "camel's nose under the tent" would be more appropriate.

It is highly instructive to check out the original proposals and promises for things like the income tax, social security, Medicare, military spending programs, regulations, anti-smoking laws, and many others. I'm sure you can think of your own examples. Then compare these promises to what we actually wound up with.